mysqli is an improved extension, I prefer it on all my projects. I was surprised to see WordPress didn't use some bundled database abstraction layer; although those typically create more overhead, with such a large userbase I thought it would be in there. Oh well.
It wasn't very difficult, and seems to work 100% so far.
I haven't really created patches in the past, but here it is - apply using "patch -p0 -u < wordpress-2.0.5-mysqli.patch" in the root wordpress directory.
Here's the patch: http://michaelshadle.com/projects/wordpress/wordpress-2.0.5-mysqli.patch
Any comments feel free to share here: