Frankly I don't think we can have an iPad 3 with a high resolution screen without an upgraded SoC. Such a machine would need a faster GPU and more bandwidth to memory. So if you look at this as October being a month and a half away it really doesn't look all that impossible that an A6 based device could come this year. At least from a hardware standpoint.
The A5 could easily drive double the resolution, you wouldn't need a faster SoC for that. I'd even go as far as saying the A4 could possibly do it, as long as you only render the iOS UI at native resolution, and switch to 2x mode for games and other programs that are graphics heavy. Memory bandwidth is hardly an issue, and not actually dependent on screen resolution that much on mobile GPU's such as the A4 and the A5, because they use tile-based deferred rendering (ie: ideally, all data required for rendering phases after T&L come from ridiculously fast on-chip buffers). Of course memory size and bandwidth requirements will be higher if applications start using double resolution textures, but that's only relevant when you are pushing the limits, ie: in games and such. The only thing that matters for rendering the iOS UI on double resolution is fill-rate, since geometric complexity and texture sizes don't increase much rendering the same UI at double the resolution.