Paul Asadoorian and Larry Pesce, authors of Linksys WRT54G Ultimate Hacking, wrote in with this excellent hack:

Over the past few months I’ve been contemplating a few projects for some WRTSL54GS routers with OpenWrt, however I really need these to have a high gain antenna on the WRTSL54GS. As you may recall, this model has a fixed antenna, with no option for adding one. I decided that I needed to fix that “design flaw”.

Pretty nice; it lets you reuse antennas from some other Linksys gadgets.

Adding a removable Antenna to your WRTSL54GS