Well, I've been thinking about the hardware a bit too. The path I'm starting to look down is something like a plain old 8051 microprocessor ($2) as a signal processor that sends a few bytes to the device that does the displaying/logging/whatnot via an rs232 connection (use a usb adapter if you have usb). I'm thinking of this approach personally because I have an old serial based palm pilot, and I'm sure there are probably a few pallets of them for dirt cheap somewhere. And it has display/touchscreen and other functions built in.
Just thinking out loud here but the flow would be:
8051 boots up, waits for init command on serial port.
palm (or laptop, or cellphone or pda or ardunio or ???, henceforth referred to as "palm") sends and "I" (for init) followed by a few bytes to tell the 8051 what to use for voltage thresholds and direction for decyphering the VSS and injector pulses. The 8051 will use a couple of its A/D converters to connect to the vehicle.
8051 says "OK" and starts polling the injectors and vss
Once a second the palm will send an "F" for fetch.
This will trigger an interrupt on the 8051. The 8051 will respond with several bytes:
1st 4 bytes: total number of sample cycles since last fetch
2nd 4 bytes: number of sample cycles where the injector appeared open since last fetch
next 2 bytes: number of vss tics since last fetch
and then the 8051 will clear all counters and resume
The palm will take the counts and update the various trips/instant data based on adjustment factors (vss tics/mile, how much fuel was actually delivered) and update the display, and draw pretty pictures if you like
In this way, all the configuration data is kept in the palm, and you can save/load specific parameters for specific vehicles, and probably hotsynch your fuel log with your computer.