I decided to cut up another PAYG O2 SIM for my testing. I did eventually get it to work with two O2 SIMs after much fiddling, switching on and off, then finally trying other phones, I first got it working in my Samsung C3050 (which is locked to O2), after that I moved it to my N900 and got it working in that calls and switching worked in 3G-only mode, I didn't test data though because it would cost money on those cards (which is why I wanted the adapter in the first place), so yes, I can confirm it (the magicsim 28th) does work, although incidentally while testing I discovered 007-mode doesn't work in my 5210 or 8310.

I still haven't been able to get it working in conjunction with my Three SIM though, I think it is something to do with Three using an STK for something (I'm not quite sure what) because the only time I managed to get service on Three through the magicsim when it was in STK-mode, but obviously I couldn't switch. Well, after I had the success with the two O2 SIMs, I tried again with an O2 and Three combo, but after much trying and switching the SIMs between the slots the N900 wouldn't even recognise there was a SIM there. I'm going to have to give up on it again for now.