Did not think about this till I got the press release, but sounds like a must have. Set custom sounds (think ringtones) for email. The use case is pretty clear. If you get an email from an important client, you may want to have a more urgent ringtone. Friends and family something else, and set default to a blank sound. Nice.
The app is called mailtones. You can find it in the itunes store here. Its $2.99, ave price.
I have not tested it myself yet, but its not like this should be technically difficult, its simply a good idea. Surprised, actually not, that Apple has not provided this by default.