Please don't, that's dishonest and intrusive. And won't be effective anyway when there's ASIC.
+1
"Stealing" users' computing power for practically nothing makes no sense at all. People who can and will donate something, will do it via your address - not by lending their computers for you.
Also, for crying out loud put some content to the site! Now it's just a black login screen (and black registration screen). How would anyone know what they're even registering for? Few screenshots, some explanation what's going on and such for the starters.