Hi there!
I have the ThinkPad 600X, which doesn't have internal WiFi.
My solution was to purchase a CardBus WiFi card, and it works fine.
However, it sticks out quite a bit from the slot, and makes it awkward to carry around without removing it first.
I was wondering if there is a WiFi CardBus adapter that stays flush with the laptop.

Note on the Cisco Aironet 340:
I have pulled a Cisco Aironet card from a PCI adapter. This sits flush with the laptop, as the antenna is external. However, it does not work with Windows: I can auto-connect to my access point, but keeps disconnecting and reconnecting every 10 seconds or so. This is with the Microsoft driver. With Cisco's driver, the card freezes the computer when inserted, and when removed goes back to normal. Cisco says it's an interrupt problem, but I have configured the interrupts and memory range manually, to no avail.

That would never work, since the antenna then would be inside the PCMCIA cage.
You'd have no or almost no connection whatsoever.
You could do some DIY.
Find an antenna wire that you could install under the (plastic) palmrest, or rather, inside the LCD.
Then lead the wire to the pcmcia cage.
You need to open up the wifi card, and cut off the outside (sticking out) part.
Then connect the internal antenna wire to the connector in/on the wifi card.
I've never tried this, but that seems the only solution.
Alternatively, there are USB wifi dongles, which are smaller than the PCMCIA cards.

I don't know that machine.
Maybe you could replace the modem card with a wifi card, using the antenna as described above (but you'd need 2 antenna wires now).
It seems to be possible, check this post: viewtopic.php?f=6&t=90827

Have you tried 3Com XJack? XJack is a retractable antenna: it protrudes two centimetres when in use, and hides inside when unneeded. There are three CardBus models, all with Atheros chips.

3CRPAG175 802.11a 802.11b 802.11g AR5001X+
3CRPAG175B 802.11a 802.11b 802.11g AR5006XS
3CRXJK10075 802.11b 802.11g AR5005GS

edit 2012-09-28 to correct mistake: 3CRPAG175B is a AR5006XS device. not AR5004X.

I found one for $12 on Amazon (WiFi A/B/G). I didn't know they made XJack products for WiFi.
Cool, thanks so much!

It slipped my mind that these existed. I use to love Xjack cards, even the modem ones.