Is there a temperature monitor/device which I can attach to the fermentation vessel that will post temperature data back to a PC via Ethernet or wireless?
I don't know of any, but if you like playing with electronics and programming, you could use an Arduino with an Ethernet Shield, and an Off-the-shelf temperature sensor. Doing that, you'd also need to set up a server process on your computer to listen for reports from your device, but there are tutorials for doing that sort of thing for just about every computer language out there.
I came across this site yesterday using an Arduino like Baka suggested.
It's a little involved, and it would require you to beef up your coding skills, but the step by step guide gives quite a bit of detail. Also, this is a fermentation controller that also tracks and graphs temperature, so it might be possible to scale back the development.
There's also an AMA on reddit with the designer.
If you aren't a programmer, then you can start building your system around a BCS460. I am running one, and it works like a charm. Mine controls my HLT, BK, Mash, and Fermentation temperatures. A little costly though if you only plan to use it for fermentation temperature, you can access it via your home network while you are at home or away.
I read in Wireless something about iGrill. This is a bluetooth thermometer (designed for cooking) which hooks up to your iPhone/Pad. I couldn't see the point of it myself but it might be interesting to you.