Keep that part of the whole sim card, put a sticky tape at the back and bingo you can place your iphone sim in it to be used in other phones or to gather your old contacts from your previous phone.
Actually, I did throw mine away and after that I bought an adaptor from eBay, but it did not work at all. When my son got his iphone 4 last month, I decided to keep that middle part of the sim and we have tried it a few times with our other past phones and it works flawlessly.
Nice idea. I would have liked to have done that, but both my iPhone and iPad came with the SIM already installed, so I bought an adapter. But the adapter was a smidge too thick and I eventually got it sorted by sanding it down, but it was a lot of work. Your idea is so much easier.