I'm not sure if this one is a bug or a feature change. I'd like to think it's the former because it's very annoying, but who knows. I'm running Vista RC1 (build 5600), so we'll see when RC2 (or whatever the next publicized build is) comes out.
Here's the synopsis: When browsing directories in Windows Explorer, if you press the Back button, the first directory is selected instead of the one you were last on. This can be rather annoying if you're used to traversing directories with the arrow keys. For instance, let's say you're at the root of the C: drive. You arrow down to the C:\Users directory and press Enter. Now, press Alt+Left (or click the Back button). The first thing you'll notice is that the C:\Users directory is still highlighted, as you would expect; however, if you press the down arrow, you'll notice that focus is moved to the second directory, instead of the one below C:\Users. Maybe it's just me, but this is contrary to how it should work.
Resolved in Windows Vista official release.