git rebase likes to fail miserably on Mac OS X Lion

On 4 different Mac OS X Lion machines, rebasing my commits (I currently 
have 14 of them) yields either of the following _consistently_ across 
varied repositories:

     fatal: Unable to create 
'/Users/joshua/src/project/.git/index.lock': File exists.

or:

     error: Your local changes to the following files would be 
overwritten by merge:

^^^ There are no local changes when the rebase begins.  This is caused 
by the rebase.

I have tried both Git v1.7.5.4 and 1.7.8.4.

I use msysGit for the majority of my Git usage, and I do not run into 
this problem there.

Is there anything to be done?  Right now, the only workaround I can 
think of is to cherry pick changes one at a time as a fake rebase.  Ick.

Thanks.

Josh