This is the most recent summary of state tax withholding provisions that I have seen:
http://www.cigna.com/professional/pdf/CPA_iidw0201.PDF

I'm pretty sure some things are out of date. Does anyone have a more up to date / complete list or link?

(Interestingly, if you look for this on the Cigna website, the link is inactive.)

I'm not sure where I got this from, but here's one:

http://www.sisterstates.com/