Will this work for you? you can buy it for $49.00. I have two of them at work, no one has written I2C sniffer code for it that I know of yet but it works for many common tasks in the lab.
the PICkit Serial Analyzer from Microchip
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en028600
David