1) Does anybody even know the difference between an A4 and A5? Is A4 single core and A5 dual core? I honestly don't know. Like others said, is it just some marketing thing? Intel kept calling stuff "Pentium" for years even though it was a fancy name for a 586 processor and eventually they went to 686. I have no clue what all the Core i3, i5, i7 crap means. Don't all processors have cores?
A4: 1Ghz single core, Arm cortex A8 architecture, imagination SGX535 graphics core.
A5: 1Ghz dual core, A9 architecture, imagination SGX 543MP2 (dual core) graphics.
The CPU went from single core to dual core - best cast, double the speed as you have 2 processors instead of one. However, most apps will only use one core and not gain the benefit. They'll still be more responsive though, because the background system stuff (checking mail and so on) no longer interrupts the app, it can work on the 2nd core.
It also went from A8 to A9 architecture. What that means is that each core is quite a bit faster (maybe 30%?) than the old A4. So you have 2 CPUs, each one is faster than the A4.
The graphics chip went from the 535 to the 543, which is much faster, and again it went from single to dual core. Apple claims it's 9x faster, in practice it's not always that fast, but it is WAY faster. Unlike the CPU, all apps use both cores on the graphics chip.
2) Does something called an A6 actually exist, or is this like Justin Bieber promoting 6G?
It probably exists in a lab somewhere. It takes a long, long time to go from 'design complete' to being actually manufactured in large quantities, so it's possibly already finished and being tested in a tablet somewhere at apple's HQ.
3) As the last person I saw mentioned, I'm more interested in display resolution boost. The iPad 2 screen is very very good. But it's not the iPhone 4 screen. I can't imagine the level of awesome of a display at the PPI level of the iPhone 4 on the iPad. I'm especially thinking about this in ebooks.
I'd love a retina display too, but I wouldn't want things like games running at 1/4 speed (because it would have 4x more pixels to draw!) You need both the A6 and the retina display together, so it can cope with the extra workload the big screen needs.