Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I have done this with a PAYG Three sim for my iPad and it works a treat.

If you decide to do this then just take your time trimming it

I suspect that o2 might offer a swap from sim to micro sim to keep customer. but thats just a guess
 
If you register interest in the iPhone 4 on the O2 website they ask for your details so they can send out micro sims to those who tick the box to "Purchase at Launch"
 
wonder along to the o2 shop and if you need a microsim they will do a few taps on their computer and hay presto your account is moved to the new sim card, they then give you the new microsim and you then put it in your phone, remember to destroy your old sim card that has just been transfered.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.