I picked up the AT&T Tilt the other day, but didn't get another plan or SIM with it. Instead, I decided to use the SIM that's in my iPhone. Here are a few results:

• Voice calls work
• SMS text messaging works
• Voice Mail works
• Notification of voice mail is an SMS with very cryptic contents, which does not include the phone number of the caller. This weird SMS (probably related to Visual Voice Mail) notification is followed by a more normal 'you have voicemail' style notification
• GPS works, assuming you're using an application that calls the APIs (C and C# both work). None of the pre-installed GPS applications work.
• TeleNav does not work, requires $10 a month plan.
• AIM, MSN, Yahoo Do Not Work : Message says, "IM is not supported over Wi-Fi connections"
• Browsing over WiFi works

A few of the applications (TeleNav, Messaging) require a real data plan, and aren't usable over WiFi.

AT&T offers a data-only plan for around $30 a month, and another $10 a month plan for TeleNav. This requires a 2-year contract, and gets you a new SIM and a phone number which no one can call you on, since there isn't a voice plan.