Bryant over at AeroXperience chimes in about the PDC build of Windows 7:

Based on the trend we saw in M1, the thing which really shouldn’t be called the Sidebar anymore will be embedded into the desktop even further, eventually annihilating the concept of a sidebar in the first place. The gadgets might remain on the side, but for all intents and purposes, the gadgets might just stay on the desktop, ready and willing to be called to the front using a key combination (not different from what you see in Vista now: Win+Space to pull gadgets to the front in Vista). It will likely be more heavily integrated into the shell than we saw in Milestone 1. Besides these changes, I doubt M3 would have anything different for the user who takes a look. The gadgets thus far would most likely be the same, though I personally hope they switch to a better code base for the gadgets.

Homegroups might actually work by now! I suspect the Homegroups feature to be much more well integrated into the networking center as well as other functions in Windows 7 Milestone 3. Joining a computer to a Homegroup, for instance, would likely be something which could be done from the Network and Sharing Center as opposed to only being established during what used to be called the OOBE as it was in Milestone 1. It might even be possible to join a computer to multiple Homegroups , though if you only have one home, I’m not sure what good this would do.

Themes probably wouldn’t be such a discombobulated process for the user in Milestone 3 as it was in Windows Vista. As a part of the goal to make simple tasks easier to do in Vista, a number of quick thematic combinations (visual themes. Probably nothing fancy yet) will likely find their homes in the main Personalization page by way of an iconic representation of the final outcome. Customization would still be available to those who wish to choose something less standard, but the default combos would be helpful for selecting a mode on a glance, such as a high contrast mode for those who have difficulties seeing what’s on their screen.

More soon, I'm sure. I can't wait to see the build.