r420 is out and introduced the ability to update the loader from within
Just go to settings, unlock the loader (default pass is ab121b) and click the update button (Not now, try it tomorrow when a new rev would be out :p)
Thank CorneliousJD for the file hosting
I checked the source as I'm not at home to test it, and it appears that there should be an update_path option at the loader to give the directory you want to update ;)
The default one is of course "SD:/apps/usbloader_gx/"