As you can see, I did achieve success with the cutdown SIM -- but things did not go entirely perfectly. The cutter works with a pre-measured slot. You slide your normal SIM into place and then squeeze the cutter to pop out the micro-sized version. That went flawlessly. But the newly cut version of the card itself was just ever so slightly too big.
I ended up having to trim it down a smidgeon at the bottom with scissors, a process that made me quite nervous -- and all I had at stake was a $10 SIM card. You might feel differently if you were cutting down your day-to-day SIM with an expensive data plan on it.
After testing it in the iPhone 4, I needed to return it to my 3GS. The review version of the cutter was shipped with 2 adapters, similar to the microSIM adapters I tried out a few weeks ago. Although initially it did not fit the adapter, my scissor trimming allowed it to snap in -- just as it did with the iPhone 4 holder.
All in all, I found the cutter easy to use and hard to mess up. My only concern is the slightly-too-long template that needed a little shortening, but that seemed to work out well in the end. The cutter costs $30 and can be ordered from the MicroSIMCutter website. It can be a great solution for anyone who buys a phone off-contract and wants to add AT&T service, who wants to transfer data service from an older phone to an iPad, or who wants to share a SIM among several devices.
The use cases are, admittedly, a little obscure but if you need this option in your life, it's nice to know that a simple solution is out there that can be easily used even by klutzes like myself.