BTW your model of Linksys is rather infamous. Linksys are a great brand but the wrt 54's were lemons. I'd suggest port forwarding ... check your model here http://portforward.com/routers.htm

I can't remember whether 4.8.1 had UPnP support in any case. If you want to see a certain fix for your Linksys but only for the very skillful .. see the paragraph about Linksys under the UPnP secton here Connection problems? Check here first! (click on link) & look for the link called 'WRT54G - Fixed my problem, Followed these steps and it works!'