Unfortunately, at this point, there isn't.
For the time being, you're left with one of two options, each which delete only one message at a time:
1) tap the EDIT button then use the minus+confirm DELETE
2) swipe your finger across the subject/teaser of the message you want to erase and confirm DELETE
Hopefully a multiple delete or delete all feature is coming in a firmware update to the iPhone.
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You can if you configure your account as an IMAP account then on your PC mark all messages as read or delete them all. I wrote an article on how to do this. This work for all Gmail Accounts, Google Apps for your domain, etc..
http://www.louish.com/2007/08/
Of course you can do it from your PC, but not FROM the iPhone.
It is hard to believe that a company such as Apple, who spends so much time delivering great products, screws up on something as basic as e-mail. However, they have managed to do it. By not providing a method to delete multiple e-mails, Apple has introduced the most inferior e-mail client to date. Apple, please correct this most grevious oversight (To be honest, it is hard to beleive this is an oversight. If deliberate, it makes me wonder why it was left out).