I'm planning in using my web cam as a security cam at my front door and I was thinking in using a pc downstairs to see my web cam over the net, but I have keep both pcs running at the same time and the need of extra software as well, so i decided to use a Cat5 to run a Usb connection over at least 25', I've been searching a little over the web and I found a couple of ideas to connect Usb peripherals over Cat5 successfully and I went over the subject, but I realize that i didn't want to cut the cat5 and solder the usb connections to the cable, so I decide to build a couple of Usb dongles (female and male) to cat5 and the cool thing is that they can be used on existing Ethernet network wall connections as well. I know that I might be pushing the limits but it's worth the try.
I wouldn't use this devices for Data tho, I understand the there are plenty of connectors out there that have baluns (which I don't quite know how they work), to send and receive Data signal, so I wouldn't risk the try but for peripherals such as web cams, mouse or keyboard, and sound signals would do just fine (even S-video).
I would highly recommend using a powered Usb hub for the distance mentioned above, in regards of some of the limits for this type of configurations:
Usb limits are up to 15' (4.5 meters)
S-Video up to 50' (15' meters)
I got very useful information from Tony's Cafe blog, on distances limits and some other valuable info, you may want to take a look at his site: http://girasoli.org/?p=58