I was thinking about writing an app to do this.
Barcode scanners generally either send their data to the PC over a serial port, or by pretending to be a keyboard (called a keyboard wedge). I think that the best idea is to have the phone on the same wifi network as the PC and then send the data to the PC over the network. As 'alostpacket' says, the phone side of it probably won't be too hard...just a matter of calling existing programs to do the scanning and then sending the data over the network.
In the past I was able to send info that I typed into my phone to a PC serial port using Telnet and some open source apps on the PC. So the serial port type scanner would basically be done with that approach. I think that in order to make a keyboard wedge out of the phone I'd probably have to write a custom app for the PC side to gather the data coming over the network and send it out as keystrokes somehow.
I think I will have some time in a couple of weeks, maybe I can get into it then
