As Joerg Wunsch wrote:
Using -D (for a date) and -r (for a branch tag) together is one of
the really weak points of CVS.
I spoke too soon, it seems. It appears there's been something done
about that deficiency in CVS within recent years. I could
successfully update a checked out tree to a date within the branch,
and it appears to me the result makes sense (when comparing it to the
current state of the 1.6 branch).
Galen, try checking out the WinAVR-20100110 tag from avr-libc's
repository. If you build it on a Win32 machine, and perform a "make
install", I'd hope the resulting installed trees will be the same.