I've got my builtin-checkout implementation at:
git://iabervon.org/~barkalow/git.git builtin-checkout
This may be interesting to some other people at this point, despite it not being relevant to 1.5.4, because (a) it includes a slew of small preliminary patches to other stuff that might be useful for making other things builtin and (b) these changes reflect my debugging of my naive implementation of builtin-checkout, and may suggest things that might be wrong in existing builtins that do multiple operations in the same process.
I'll actually send patches to the list post-1.5.4 of all this stuff.
-Daniel *This .sig left intentionally blank*