Unfortunately, excess skin cannot be exercised away. Weight lifting can build muscle that can help "fill out" the extra skin, but doesn't make the excess skin disappear.
BTW: I recommend checking out the Body Image and Issues After Weight Loss
http://www.3fatchicks.com/forum/forumdisplay.php?f=219 for several threads on skin concerns!

(Including the sticky that Heather posted a link to!)