I currently go through Cingular, using Decca's old Sony Ericsson phone with my cingular SIMM stuffed into it. Contract has long expired, it's a long since discontinued voice and data plan that costs very little per month. So. No new contacts or service plans for me.
This limits me to any GSM phone that's available unlocked on ebay or other online merchant that won't require signing up for a new plan.
I was looking at a Motorola Razr V3 but the reviews are scathing.
My most important features:
takes a GSM SIMM card
small, robust, handles being dropped and treated like crap.
My least important features:
quadraphonic surround sound ring tones
mp3, video, blah blah.