TEXT ONLY PLEASE
I use pine on my own personal linux server (for mail, and notes, any anything else of importance). I can read my mail from pretty much anywhere.
A web-enabled phone would be useless to me, as would one with its own proprietary email system. The only way any such device would be to me would be if it supported TELNET, and a reasonably correct terminal emulation (vt100 would be fine).
The 'one true format' for information should be plain ascii text, possibly in some parseable form. There are damn few places that wont work. Graphics and fonts are great, but should never be REQUIRED to read information, but should always be extra options (optional to the READER, not the sender)