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.








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.