In short: works like a charme.

I was looking for a USB wifi adapter that could provide me a better range then the built-in wifi of the X61s. Took a quick look through the net and the kernel config and decided to buy the ZyXEL G-220 v2.

After building the kernel module (zd1211rw) in the 2.6.24-rc7-git4 gentoo sources I only had to emerge the firmware (net-wireless/zd1211-firmware) and everything was working fine. HAL/udev(?) seems to load the modules and knetworkmanager connects to the networks.

And the network strengths are much better (either that, or the display doesn't work and just shows 100% for all of them ;) ) 

update: The latter seems to be the case, signal strengths are always 100% ... hmm ... have to solve that.

update: On 2.6.25-rc5-git1 the signal strength display is working correctly, the sad thing is: they are even lower then those of the built-in antenna. I think I'll return or sell it.