I have an X31 which came with the Intel Pro/2100 Centrino 802.11b MPCI card. FWIW, in Windows, I
had pretty good luck with this card - performance was pretty good, I had no problems with dropped connections. The connection manager software (IBM's software) was decent. Range was better than any of the PCMCIA/USB adapters I've tried (D-Link USB (not so hot), USR PCMCIA, SMC2532 PCMCIA (great card if you use an external antenna(s), Intel PCMCIA), probably because of the built in antennas.
Because of the lack of Linux support with the 2100, I decided to get a Cisco 350 series - with 100 mW (20 db) transmit power and greater receive sensitivity ratings, I thought I'd get better range out of it as well. Well, I don't get better range, and so far I haven't been able to get it to work with any Linux drivers - 2.6.1 kernel, and the drivers off the Cisco website. Hopefully after spending more time on it, I'll have better luck. At any rate, the WinXP Cisco software is great for this card. It allows you to adjust all kinds of parameters, including specifying diversity/pri/secondary for transmit AND recieve, and all kinds of low level wireless settings (power, rts/cts settings, etc). But range was a dissapointment.
I don't really care about the WiFi speed, as DSL/T1 is usually the bottleneck in my WiFi connections - and without downloading DiVX, who cares about 1 mbps from the Internet... If I really need to transfer files quickly, I can always go wired - hasn't been a problem yet tho.
I'm curious if anyone has any feedback on the Atheros based IBM MPCI cards range compared to the Intel Pro/2100 and/or Cisco 350 series? Anyone noticed a significant difference after upgrading? Like I said, G isn't really much of a selling point - and A is even less of one for me, but range is a big issue... anyone whose upgraded care to comment on that?
FYI if anyone is considering PCMCIA and wants long range, I can highly recommend the $58 (@ Amazon) 200mW SMC2532 PCMCIA w/ a detachable antenna + an external $16 5.5 dbi antenna (Ebay RP-MMCX - make sure it's reverse polarity!!!) - it's a potent PCMCIA combo for cheap. The Senao's sound great as well, but you have to either go external antenna or built on antenna, can't do the same on one card. In a perfect world I think I'd go with the Senao 200 mW MPCI, but IBM has to be nazi's with their BIOS...

There's a $125 15 dBi omni 40" antenna I've been eyeing (Hyperlink) which I think would be great fun to attach to the roof of my jeep along with the SMC... mmmm free internet
Rob