They currently allow (split by 10 for better comparison):
320:240 (13.3:10), 480x320 (15:10 ratio), 800x480 (16.6:10), 854x480 (17.7:10/16:9), 960x540 (17.7:10), and then 1280x800 on tablets where 48 of the pixels are for the bottom bar, so it's more like 1280x732 (17.4:10).
I would be easier to do "pixel perfect" design if Google would at least not use all these resolutions that have little to do with each other. It goes to show that Google doesn't put much thought into these kind of design details.
This is the kind of "standardization" I'd like to see Google bring to Android. Would it be that bad to deny a manufacturer the use of a 854x480 resolution over a 800x480 one? or whatever they choose to be the standard? I don't think so, but they don't seem to care much about it.
The 960x540 resolution was probably unnecessary, too, considering we're about to see 1280x720 resolutions coming into market just 6 months later. If a manufacturer decides to make a 1366x768 display later, should they allow them to do it? They shouldn't because the improvement is marginal and developers have to support one more resolution with little benefit, but they probably will allow it.
Maybe Google thinks, as I do, that designers should be able to make their stuff look good in a variety of resolutions. Desktop/laptop computers have a range of resolutions and aspect ratios, but people have been making UIs look good on them for years.
1. There aren't really that many native ratios. Realistically, you're looking at 4:3 for CRT's, 5:4 for non-widescreen LCD's, and 16:9 or 16:10 for widescreen LCD's. So that's not as bad when creating a full-screen application, because you can also letterbox the 16:10 to 16:9 for full 1080p video.
At this point, if I were a developer, I wouldn't even care about 4:3 resolutions, and just have 5:4 and 16:9.
2. Desktops and laptops, in general, have a lot more pixels to deal with, and are usually farther away from the user's face, so there is a bit more leeway when scaling your UI.
