Here's something I've been looking for for months but for some reason couldn't find on the web. Maybe I was just using the wrong search terms. Now, by chance, I find it on the O2 site while researching the new iPhone 4. It's the SIM-caddy, a carrier to take the micro-SIM and allow it to be inserted into the larger SIM slot on older phones and, say, the MiFi.
This is going to save me money. When I took out the £15-per-month/3GB iPad service I decided to keep the old £15 contract going because the MiFi needs a large SIM. Otherwise I wouldn't be able to get mobile internet on my MacBook Pro. Now, with the SIM-caddy I can cancel the old contract and simply transfer the micro-SIM to my MiFi (which then provides internet to both iPad and MacBook Pro) on for those occasions when I'm travelling with the laptop. Result: A saving of £15 a month.
Since I always keep my last phone as a backup, just in case of theft or damage, I will pack one of these adaptors just in case. This doesn't do anything for the problem of finding a micro-SIM in a country that hasn't yet been prodded into action by the arrival of the iPad. I intend to practice with a sharp knife to see if I can cut down an old SIM.
According to the O2 site, just call into any O2 shop and they will give you a free SIM-caddy. I suspect the other carriers will be offering them also.
AFTERTHOUGHT: Having congratulated myself on working out the above solution to making my iPad data SIM portable, I suddenly realised why it won't be possible. The data micro-SIM for the iPad is restricted to the specific IMEI number of the device. I thought this odd at the time I took out the contract because it has never been necessary before. So, I presume you need to get the IMEI number of the contract changed if you want to swap devices. This is a further restriction the carriers have slipped in almost without anyone noticing. I assume they've done it specifically to stop family members sharing SIM cards and, presumably, to stop sharing (via MiFi or through USB modem) with a laptop. But why? Surely the contract is for a maximum number of gigabytes per month (in my case 3 GB). What concern is it of the carriers how you use that allowance?