It appears it doesn't work. When you log into the APN then it knows what type of device you are and refuses to allow you data access if you're not a Sidekick device. Search around for iPhone and the T-Mobile Sidekick plan to see lots of complaining about it.

I'm unsure whether this is because they're on a regular plan with the Sidekick data plan bolted on or because they've got a Sidekick SIM inside a non Sidekick device. You'll have to look into it further.