Cross warrantees their pens for life. I squashed one of their mechanical pencils under the wheels of a rolling desk chair, and they replaced it with a new one. Their warranty covers pretty much everything except dropping a pen on its nib. If the nib isn't actually damaged, they should fix it for you. They charge $10 for shipping/handling.
Here's the link to Cross Repairs Info:
http://www.cross.com/customerservice/repai...at_name=RepairsIf you follow the link for Fountain Pens, there are details at:
http://www.cross.com/catalog/refilllisting...eplacement+NibsYou can also buy a new nib from them. You didn't say just which one you had. The Cross II nibs run from $25 for stainless steel , $35 for 23Kt gold plate, $70 for 18Kt gold. Shipping seems to be around $7.95. It looks like the nib includes the whole front section, so it seems to be basically unscrew the old one and screw in a new one.
Good luck. Let us know what happens. I'm always interested in how different companies handle customer service.