I've just posted a how-to over at wrt54ghacks.com on adding a removable antenna to the WRTSL54GS versions 1.0 and 1.1. This modification will allow you to use all manner of antennas with RP-TNC connectors with your router.
Check out the posting here.
As always, comments are welcome.
- Larry "haxorthematrix" Pesce
larry /at/ pauldotcom.com