Can I insert my t-mobile sim card into a iphone?

I currently have a blackberry bold 9900 but my mum is due for an upgrade soon so she won't be using her iphone 4 anymore. She said I can take it but i'm not sure if my sim card will work in there. I can't get the iphone through contract atm because my current contract is for the blackberry. I'm assuming that I will have to get the iphone unlocked with my sim in it for it to work but does anyone know for sure?

Yes.

Yes, only if the iPhone is unlocked for any GSM network.

No it wontcgo in i'm afraid, the iphones use a special sim called a micro sim.
Also you can't get an iphone unblocked unless you buy them from the apple store
Hope this helps

You will be able to use your SIM Card with the iPhone if you unlock the iPhone and cut down your SIM Card to fit in it (no problem).To get your mom's iPhone unlocked, just go to one of the big online vendors like http://www.cellfservices.com/ and activate it with your SIM Card on iTunes then, no big deal.

Yes you will be able to. But each iPhone takes a Micro SIM card (apart from the iPhone 5 which is a nano SIM card) and the micro sim is just slightly smaller than a normal one, for you to get your SIM card in your mums phone, you will need it cutting down to a micro one which most phone shops will do this. And if you ever don't want the iPhone in the future, then you can get an adapter to put a micro back into a normal one.

(By the way, a nano sim is smaller than a micro)