This may depend on the data package. The required data package with the Sidekick probably gives you unlimited IMing. On may last two T-mobile phones IM's and emails have both been charged at SMS rates when using the built in messaging apps.
I don't mean that AIM uses SMS to actually send/receive messages, rather that each IM is charged at the SMS rate (also each email sent or received via the built-in client is charged that way).
That's why, for example, it's cheaper to use T-mobile's $6 unlimited web access with Gmail's mobile app (which apparently uses http(s)), than to use T-mobile's email client and an SMS package. That of course doesn't apply to a phone with a required data/text package like the Sidekick.
