We've had a look at 4042.Lab06_N.030909-1709 already, so I thought I'd show the difference between it and its counterpart, 4042.main.030905-1800.
Looking at the two builds side-by-side, you could be forgiven for thinking they're completely different OSes. The MAIN build is quite clearly still using Milestone 6 code; the boot screen, welcome screen, wallpaper and shell are almost identical to 4040. The LAB06 build however, 4 days younger, is completely different visually, and is much more stable and usable.
Evidently this is around the time of the changeover to the newer Milestone code. Such is a massive engineering effort as all the labs are synced literally overnight as the new code is brought in. Anything that hadn't been reverse-integrated into main for the milestone is dropped. This explains why the LAB06 build is missing the experimental 3D features found in the MAIN build and other M6 builds; everything not stamped with approval for inclusion in the Milestone 7 code is shelved. That's not to say it mightn't be added back later - PDC03 showed us 405x builds with the "3D" view option put back in, and the 3D views are up and running again in 4066.
Without further ado, here are some shots of 4042.main. It's almost indistinguishable from 4040, so I left Aero Glass on its default opaque mode so the shots at least look a little different =)