Spike and Buffy gender/body swap. Body swap preferred, but either one. I just read a fic on this topic, but I was wondering if there were more because I've never seen them until today.

theonewhoneversleeps
There is a really great one called Impossibly Delightful Flesh by Sadbhyl
I'm not sure if it's archived here but It's posted at bloodshedverse and the buffyfanfic archives as well.
One of the best read's by one of the best authors!
Hope you enjoy...

Impossibly Delightful Flesh at Sinister Attraction

http://www.sinister-attraction.net/stories/s/impossiblydelightful.htm

Coalitiongirl has a list on LJ of bodyswap fics. She's not on my friend list so I don't have the link handy, but one of the other ladies here might.

Walking a Mile by Ariel Dawn - is really good. It's on Elysian Fields

http://dark-solace.org/elysian/viewstory.php?sid=1742&warning=NC-17

http://coalitiongirl.livejournal.com/74750.html
http://cordykitten.livejournal.com/340740.html

Two rec lists from February and March this year. With a link to the ficathon (but several pairings possible.)

Don't know if you're still looking but Arina Summer has a few on fanfiction[dot]net...I think she may be in the process of editing and putting them up here though.