Well I would recommend Cingular because you can get any GSM phone imported and use it. If you need a phone that Cingular sells, I would look at the Blackberry 7100g or Nokia 6620.
If you want an imported phone, I would look at the HTC SDA.
Link -
http://www.mobilebee.com/PhoneDetailsID391.html
The PalmOne Treo is also a good phone, I had one until I just switched to a Sony Ericsson Z800i.
Blackberry 7100g for free right now.
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The Nokia 6620 is $24.99.
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Treo 650 for $299 -
Link