In a couple of blue posts, Bashiok clarifies what the changes to Inferno difficulty entail and why did they choose to go from a flat to non-flat difficulty.

As you may know, Blizzard's initial design of Inferno had a flat difficulty across all four acts, meaning that wherever you went you would face the same challenges. Apparently, Blizzard were not satisfied of how that worked and changed Inferno to scale in difficulty as the game progresses, similar to the other three difficulty levels. In his first post, Bashiok explained the main reasons behind this decision:

This is may be a good a time as any to REVEAL TEH SEKRITS! that Inferno monster levels aren’t linear any longer. They get progressively more difficult. This was really a reaction to Inferno playtesting. Our original intent was to have a flat difficulty level where you could go wherever you want, farm for items, and it’d be no more or less difficult than any other area in Inferno. This caused a few inherent issues for us, though:

It just felt wrong. It didn’t feel right to be progressing through the game and have it stay pretty much the same difficulty the whole time. It felt like a letdown to get to the final boss of the game and it be no more difficult than the first.
There’s a wide variety of players out there and we wanted to make sure everybody had something to sink their teeth into. We expect that anybody with enough time and dedication will reach level 60. But the jump in difficulty to Inferno needed to be different amounts for different people. For the crazy people they need a HUGE ramp in difficulty, for a more “casual but still hardcore” audience you want an obvious but milder increase in difficulty. So for the crazy people who play non-stop they’ll hit Act I and get a challenge, but 1 month later they’ll still have something to work on (Acts II, III and IV). For the “hardcore-casual” they will reach level 60 later and not get brick walled when they reach Inferno. They can experience some “small victories” working on Act I with the dream of maybe someday reaching the later acts.
Longevity. We know people really want goals to work towards and challenges to overcome. We made Act III and Act IV really, really brutally hard, for the most elite players only. It felt wrong to make ALL of Inferno that brutally hard.

Now, you may be saying “I thought you wanted us to be able to farm anywhere we wanted. Now we only have half as much area in the game to farm in? What gives?” Our goal is to make the loot mathematically better in the later acts without making the earlier gear completely obsolete. We feel Diablo II actually did a very good job with this and we expect Diablo III to perform similarly.

Specifically, people in D2 did Diablo runs, Mephisto runs, Pindleskin runs, Pit runs, Baal runs, etc. because the loot in Diablo is extremely random. Even though the theoretical best items might come from the later Acts, well-rolled items from earlier acts will still be better. Internally we find sometimes after an intense session of brutally hard Inferno it can be really fun to cruise through Hell Act III or IV and it’s not too uncommon surprise when an upgrade drops. We expect this to carry through to Inferno difficulty where somebody who can theoretically farm Act IV will likely still enjoy romping through Act I simply because the drop potential is still there. It’s all because of the highly random items having lots of overlap in their power distribution curves.

One has to admit it makes sense - this change will bring a much more fluid transition through the Inferno acts, despite imposing some restrictions regarding the optimal farming place. Furthermore, in a follow-up post Bashiok confirms that whatever you can loot from Act IV Inferno will drop in Act I Inferno as well. Thus, if you lack sufficient gear for Act IV Inferno you can go back to previous acts and farm it up - it will take more time (chances will be lower, but the item level will be the same) but it will be manageable.

I just wrote like three pages of a reply and the forums lost it when I clicked Preview. … *cry*

Ok what it boiled down to was:

Read my previous post. It seems like some people glossed over it.
I’m happy most of you are happy about the change, and I know you’re going to love the game.
Those few of you who don’t like it, you’ll love the game too because you’re wrong.
Item pools are not limited by Act, or Boss, or anything like that. While you’ll have a better chance to get better items in Act IV Inferno, you could get those same items in Act I, or even Hell.
Our item pool philosophy is that you can break an urn and get the best item in the game – it’s all a matter of chance. Running more difficult areas and taking on more difficult enemies will not always be the most efficient way to find upgrades.
Previously, Inferno difficulty was mlvl 61 across all of Inferno, and now it starts at mlvl 61 and ramps up quickly in Act I and ends somewhere around 65 (?) in Act IV. We’ve only increased the difficulty.
I’m aware of internal bets on how many months it will take someone to beat Inferno.
A flat Inferno of mlvl 61 had a small curb of difficulty, and once that was over you had nowhere else to progress and no reason to. That’s boredom.
Boredom doesn’t generally come from content repetition, it comes from lack of ability to progress, or ease of progression.
By having a sharp increase in difficulty in Inferno we can encourage progression without having a brick wall of difficulty.

I think that was about it. In any case, as I said, I know the vast majority of you are excited about the game, the change we made, and trying to progress in Inferno. Just don’t feel bad when you have to go back to Hell.

To answer the question if going from flat to non-flat means making the beginning of Inferno easier, Bashiok responds with the following:

"Yes, the Acts each get significantly more difficult than they were before when it was all one level.

Sorry, to clarify, the beginning of Inferno is not any easier than it was before. We’ve only made Inferno harder."

Source: Battle.net

Bashiok on the non-flat Inferno: 'We've only made it harder'