How did I forget about Brat's fic White Lie?!?!
The one below is a WIP but it's over twenty chapters and a pretty good read.
Remember Me *Unfinished*
~ Author: Margie
Summary: Buffy and Spike have been best friends since they met in college nearly 10 years ago. Spike needs Buffy's help, asking her to attend a week long business trip as his pretend fiance. After some cajoling she agrees. During the trip, Buffy has a mild accident. Spike discovers his problems have only just begun when Buffy wakes to the belief that she really is Spike's fiance!
http://www.sinister-attraction.net/stories/m/rememberme.htm