It's strange to see that Gmail's mobile app still doesn't work with Google Apps accounts. The only option offered by Google is for Blackberry users: "if you currently use the Gmail for mobile application on your Blackberry, you'll need to download the mobile application for Google Apps as well. These are two different applications. A red M icon will appear for Gmail and a blue M icon will appear for Mail by Google (Google Apps)."
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. email@example.com)
* 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.