ArsTechnica posted roundup of changes between Windows 7 build 7000 and Windows 7 build 7057. Here's a bit:

There is almost three months difference between the two builds: the beta was compiled on December 12, 2008, at 2:00pm and the leaked build 7057 was compiled on March 5, 2009, at 8:00pm, so there's a lot of work to cover. This post is about tracking every noticeable visual change; although I do mention some fixes here and there, I'm mainly focusing on tweaks that are "tangible" as opposed to the thousands of bugs that have been fixed. As before, the screenshots are lined up like this: build 7000 on the left and build 7057 on the right, or build 7000 above and build 7057 below. Also, remember that the "Send Feedback" link has been removed in build 7057, so that's an easy way to remember if there is a window title bar visible."

Before I get going, I want to underline the by-far most important change in build 7057. It's not a critical bug fix; it's not even a brand-spanking-new feature. It's the expiration date. As you can see in the screenshot above, build 7057 expires March 1, 2010. What this means is that Microsoft is getting very close to readying the Release Candidate build. Very close in fact, but as you'll see, it's not quite there yet.