Had identical issue with Msysgit v1.7.2.3, the latest version as at 29 Sep 10, and wanted to share my findings here (Google turns up several cases, but no solutions).
Trying to do "git svn rebase" on a repo (that has this has worked on plenty of times in the past) consistently failed with a "fatal: write error: Invalid argument" after a certain number of commits. The sync would then revert to the beginning again.
I believe this is a bug in Msysgit relating to large(ish) binaries and available memory (on a Win XP SP3 system with 4GB RAM and plenty free HD space). The remote system was the DotNetNuke SVN repo on CodePlex (https://dotnetnuke.svn.codeplex.com/svn).
Initially it was choking on a 330KB "CHM" file (~212th commit, r52261). It consistently did so, even after disabling Avast AV, Google Desktop, etc and verifying that there were no other processes with locks on the repo folder. After a reboot (but opening Outlook, Dreamweaver, etc), it then was consistently and repeatedly failing on a ~15.3MB DLL (~416th commit, same revision).
Finally, after another reboot, disabling Avast, Carbonite and Google Desktop and running no other programs, the sync worked first time.
This seems to point firmly to my conclusion that it was an available memory issue, probably linked to the presence of a largish binary and large number of commits in the revision. Note that I also tried "git fsck", "git svn reset xx" and tweaking the "packSizeLimit" / "usedeltabaseoffset" config vars, without success.
Chad - it could be that you had the same issue and it was the reboot that helped, rather than the upgrade.