If you manually entered them on the iPad then you can't (to my knowledge). If you synced them over from your computer then there is a cumbersome solution.
Open iTunes and click on the iPad in the left sidebar. Go to Info, scroll down to Sync Mail Accounts. Uncheck all the accounts except the one you want to be first in the list on your iPad. Sync. Go back to iTunes and check the account you want second in the list. Sync. Repeat until all accounts have been synced in the order you want them to appear.
Hopefully 4.0 adds the ability to rearrange the accounts from the device itself along with the other mail enhancements.