I have a bike computer that performes twelve different functions and cost all of $10.  

Seriously, this kind of feedback can't be expensive, why aren't more automakers including it?  (I say 'more' because from googling around I see that a) it isn't a new idea at all and b) some non-hybrid cars already include it, I know my friends BMW does and it's six years old)

I wanna buy one for my civic, if I find an aftermarket source of any kind I'll let y'all know.

BTW, my bike computer says I get "excellent mileage" (say that with a Rain Man voice)

See ScanGauge (seems to be sold out now but coming in May). ThinkGeek.com used to sell these for $129 or so.
awesome, thanks!
I once had a 1993 Lincoln that had both current MPG and cumulative MPG. It was a real eye-opener on bad driving habits. By using the current guage, I was able to get my cumulative mileage up to 22 MPG on the beast.
My trip computer cumulative MPG on the Passat has often read over 48 MPG, and sometimes over 50 MPG.  The gauge reads a bit higher than the real economy, but it's still amazing to see such numbers from a 5-passenger car.

Haha - years ago I was riding in a friend's Lincoln Continential (one of the largest cars around before the SUV craze), and it had such a thing.  We were driving back from something one night, and he looks down and says "Look - we are getting 39 mpg".  Of course we were going downhill at the time....
Or this one for $ 27.99.

http://www.jcwhitney.com/autoparts/Product/showCustom-0/Pr-p_Product.CATENTRY_ID:2008961/c-10101/Nty -1/p-2008961/Ntx-mode+matchallpartial/N-10101/tf-Browse/s-10101/Ntk-AllTextSearchGroup?Ntt=miles

MILES-PER-GALLON GAUGE
For all cars, trucks, vans and RVs
Miles-per-gallon scale shows actual gas mileage as you drive
Easy to install on or under dash
Slash your fuel costs with this easy-to-read illuminated fuel economy-performance gauge. Color-coded zones tell you when your engine is producing the most power for the least gas. Approx. 3-1/2" diam. face. With mounting housing. Complete with Operating/Installation Manual.