This is more a brief note than anything... an Intel 2200BG mini-PCI card taken from a more recent non-IBM/Lenovo laptop will run in the slot of a Thinkpad X23 (model 2662-E5G) without any BIOS chicanery required. I did carry over the internal antennae as well, though I didn't fit them inside the lid; I just wrapped the metal clips in electrical tape and coiled the wire inside the space afforded by the PCMCIA slot. I gather that some models of X23 come with antennae already fitted, but sadly mine didn't. Mine also suffered from the wanker who had it before me destroying the thread on the frontmost of the three keyboard-retaining screws, so replacing the modem card with the wireless one was accomplished by prying the keyboard out and very slight cosmetic damage. Reception seems to fluctuate wildly and isn't as good/stable as it was when the antennae were hanging out of the case, but it's more than fine for web browsing around the house without fussing with a USB wireless adapter and hasn't actually dropped out yet. Drivers direct from Intel were used.
Whilst I was at it, I figured I might as well add more memory... it turns out the X23 has 128Mb of non-removable RAM, but can take up to 512Mb extra of PC133. So I maxed it out for fourteen quid, as XP usually runs very happily if given more than 512Mb; it should reduce the frequency of disk access after the OS has booted and the anti-virus stops fussing around.
The battery's still fairly duff, but it makes a handy UPS when it's in. And I've now seen too many problem/dead laptops to regret getting a sturdier older Thinkpad — its original purpose was just to watch TV shows on in bed, but it's a nice light machine with a chunky keyboard to type on too.