I believe almost everyone (except ASH and AB) are contracted for all of S7. Given that all the main cast, and SMG in partcular, are finding extra-buffy success I wouldn't be surprised if there wasn't an S8 - there all in a position to make big bucks doing movies and other shows, and the show isn't big enough to pays the Friends-grade salaries that it would take to keep them tied up for another year (SMG in particular).
by Stoo
Actually anyone have any ideas on how long Buffy will go on for?
Will S7 be the last, or can they squeeze a few more out of it?
However Buffy's important to UPN, and they wouldn't have gone through the hassle and cost of acquiring it from Warner just to see it go after two years; so I wouldn't be too surprised if we had more spin-offs and specials after S7