OK - another post on SIM cutting.
I just got a couple of free T-Mobile SIMs to practice on, and took the O2 SIM out of my 3GS just to compare and the two SIMs are very different!
The T-Mobile SIM has a very rectangular active area with three segments on each side and one in the middle. Cutting this to the microSIM size would be no problem.
HOWEVER, my iPhone O2 SIM (originally from my 3G), which is the one I would need to cut down has a much bigger, almost square, active area with four segments on either side and one in the middle. If I cut this down to microSIM size I'd have to cut some of the active area as it's bigger than the microSIM size.
Up till now I wasn't worrying to much about the microSIM thing even though my iPhone 4 is coming straight from Apple, now I'm thinking I'm going to be reliant on O2 sending me a microSIM beforehand and I think the chances of that happening are ZERO.
So if you're planning on taking a knife to your SIM come the 24th, you might want to take a look at it first!
Don't worry, with my iPad Ive cut down quite a few sims, nearly everyone has been different. You just need to make sure you measure the dimensions from the centre of the chip. Cutting off bits of the (metal) chip hasn't made a difference.