Are you talking about a free app or one that you would sell through the iTunes store?
This probably is not what you had in mind, but what's really missing with the iPhone is a good spam filter. I get hundreds of spam messages per day with my main e-mail accounts and it is really annoying to constantly delete these on the iPhone.
Most iPhone users who need spam filtering are forwarding their messages to a GMail account and bringing the mail into the iPhone through GMail. GMail does a good job with spam filtering.
I have to assume that there are some large hurdles with integrating spam filtering into the iPhone; otherwise, there would be a number of spam filters available.
If someone came up with a good solution for this problem and sold it for $9.95, they would probably sell millions of copies.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin