As I understand it, there is the potential for 802.11g cards to disrupt, or at least use, frequencies that are used by the military. Manufactureres have, according to rumor, been pressured not to release information that would allow users to select these frequencies. Open Source software does not provide any means for manufacturers to do this, and aparantly locking down the available frequencies in the hardware is impractical.
This is just rumor - I have seen no confirmation that there has been any pressure on the manufacturers, nor can I confirm that this potential is anythnig more than an excuse to stick to a propietary model.
That said, http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.drivers....
Lists two 802.11g device driver projects, the Prism54 driver at http://prism54.org/ and the Intell IPW2200 at http://ipw2200.sourceforge.net/.
A little searching finds the prism-based Tekram TM-802G available from pcbay.com at $22.98 shipped.
This article says that Intel is releasing Linux drivers for it's Centrino platform: http://news.zdnet.co.uk/software/linuxunix/0,39020390,39156340,00.htm
Here's an article from someone who picked up a prism based Netgear WG511 for $35 a year ago - gotta be cheaper now: http://toys.lerdorf.com/archives/15-802.11g-Netgear-WG511-and-Linux.html
So it's just a matter of putting in the effort to find one that works, even if you really want 11g speed for $25.