From Search Engine Watch:

If you’ve got a web-enabled mobile phone and a Gmail account, you can now access your mail on the run…

Gmail mobile detects the type of device you’re using, and formats your mail accordingly, meaning you’ll have a different view using a comparatively wide-screen Treo than a smaller Motorola handset. You’ll also be able to view attachments, including Office files and PDF documents.
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Another cool feature is “call to reply.” If you have saved the phone number of an email sender in your contacts list, you have the option to reply via email or by calling them on the phone when using Gmail mobile.

To access Gmail mobile, just enter m.gmail.com on your mobile phone browser.