SIM card isn't working in iPhone?

I got an iPhone 4 yesterday and inserted my tmobile SIM card. It has now been 24 hours and my iPhone is still saying no service. Somebody please help

Reboot your phone a time then follow following steps:
switch off phone
hold home button and connect to iTunes
then restore your iphone

Is the T-Mobile SIM activated? Perhaps a stupid question, but best to start with the simplest thing.

Is the iPhone you purchased either factory unlocked or an AT&T iPhone which has been unlocked? If not, it will not work with T-Mobile.

I purchased an unlocked iPhone from the Apple store last week and then stopped by T-Mobile on the way home. They activated my account on a new SIM (it uses a tiny SIM and my other phone used a much larger one) and by the time I left the store it was working.