An interesting comment from a Lifehacker post offered a way to login with a Google Apps account in Gmail's mobile application, by downloading two old versions on your mobile phone:
* Browse to gmail.com/app/v1.0.0/en/gmail-g.jar [or: http://tinyurl.com/yxnuqx] and download the version 1.0 client
* Enter your login information (your username should be your full Google Apps email address, i.e. me@mydomain.net)
* You will get an error after the client tries to pull up the inbox. Just exit out of the application.
* Now go and download v1.1.1 by browsing to: gmail.com/app/v1.1.1/L1/gmail-g.jar [or: http://tinyurl.com/2ge979]
* After it installs select 'Keep existing data' on your phone when prompted
* Now when you start up gmail it will use the saved login credentials from the previous install and work just dandy.
I couldn't make this work, but others confirmed that this trick works (for example, in some Nokia and Windows Mobile phones). The latest version of Gmail Mobile is 1.5, so the two versions mentioned above aren't very recent.