Any interest in merging projects? I've been slow in writing the UI pages themselves, but the framework is all in place on the MPGshield project. Right now it just has a VSS info page, injector info page, and instant/average MPG page. All I ask is that the code remain hardware-neutral (standard libraries where available, #define pin numbers and such) and is reasonably readable and/or well-commented, which IIRC is among your goals as well.
Here's the current code, including stuff to control an external 4 digit 7-segment display via an SAA1064 chip via I2C. The display comms are one-way, so it doesn't hurt anything if there's no chip to talk to.
http://dl.dropbox.com/u/9882625/MPGshield_0.3.pde