The reason the TREO works on the plan is because the device does not work from a server. The TREO is a basic email receiver and uses the email account that Cingular supplies to cell phones or a Gmail account like other cell phones. Blackberrys use the RIM server for all connections to the carrier before going to the email servers( BES or BIS ) which have Blackberry software working on them.
Generally the TREO is just a basic cell phone with some PDA options.
TREO users don't like to admit that but nothing but a blackberry is a blackberry