Can I use my nokia n900 sim in an iphone?

I have a monthly O2 contact with an unlimited data bolt on. I currently have a Nokia N900.
Could I cut down that sim and use it in an iphone or would I need a special contract? I know that I can't use this sim in my ipad (I tried but received an error mesage saying that I needed to arrange a data contract). This will influence whether I buy an iphone as I don't want to give up my current O2 contract.

Yes it will work! I had an orange BlackBerry contract sim with 500mb internet, 1200 minutes and unlimited texts. I bought a sim free iPhone and simply cut down my sim card and it works perfectly despite everyone online saying iPhone and BlackBerry require different plans bla bla bla! And just to reassure you I've been using my cut down BlackBerry Orange contract sim card in my iPhone for 6 months now with no problems at all whatsoever! It was honestly so convenient for me to do that and I've now simply put my BlackBerry on pay as you go so I can still top up monthly to get my BBM working but my contract sim card is in the iPhone which I use as my main phone since cutting my sim card down!

PS. You can get sim card cutters cheaply from eBay and they works perfectly. But maybe you could use a nail file to smooth out the edges of your sim card after you cut it down as it might not feel smooth. That's all there's to it!