The plan would be that it could sit anywhere on the network and it would output a serial stream with the decoded traffic. Haven't really thought about what other features it would have, just gauging interest to see if I should work on it.
To keep you from starting from scratch on this project, you may want to look at a similar open source project that has already done this.
See this site: http://dangerousprototypes.com/bus-pirate-manual/
Here's a summary of the interface features:
•Supported protocols:
◦1-Wire
◦I2C
◦SPI
◦JTAG
◦Asynchronous serial
◦MIDI
◦PC keyboard
◦HD44780 LCD
◦2- and 3-wire libraries with bitwise pin control
◦Scriptable binary bitbang, 1-Wire, I2C, SPI, and UART modes
•0-5.5volt tolerant pins
•0-6volt measurement probe
•1Hz - 40MHz frequency measurement
•1kHz - 4MHz pulse-width modulator, frequency generator
•On-board multi-voltage pull-up resistors
•On-board 3.3volt and 5volt power supplies with software reset
•Macros for common operations
•Bus traffic sniffers (SPI, I2C)
•A bootloader for easy firmware updates
•Transparent USB->serial mode
•10Hz - 1MHz low-speed logic analyzer
•Scriptable from Perl, Python, etc.
•Translations (currently Spanish and Italian)
•Enumerates as a virtual COM port over USB
•Can operate as AVR STK v2 clone programmer
•Access to PIC24FJ64 ICSP programming port