I just came back from a certain major retail computer chain, excited about my IOGear USB PDA/Serial Adapter (model GUC232A). It was the only converter they sold that would allow me to sync my (serial-only) Palm Vx.
After following iSync's help, by putting the Palm Desktop and the Palm Conduit (see the right-hand side of the web page) on my computer, the Sync still wouldn't work.
Then I found IOGear's driver index. You have to download and install the driver, restart your computer, (I guess) select the new conduit and unselect all other conduits in HotSync Manager (/Applications -> Palm -> HotSync Manager by default), restart your computer again, and reselect the conduit. Things should then work.
Hope this helps!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031012181550196