ah...i've never tried it when a message was on the screen...
I agree, I really wish it controlled something on the stereo (i like the stereo controls on the steering wheels on the newer models)
I usually leave it on Tank MPG (to berate me from performing lead-foot)
My average speed is like 37MPH (i work from home so i don't fight the commute, when i do travel it is on a freeway many times)
I use trip A to track my MPG manually although the computer does a good job of it also (just rounded to the nearest mile instead of a tenth of a mile); i do like the fact that the computer does not erase this data after filling up the tank UNTIL you drive off (in case you forget to write it down before you fill up)
I've noticed that the computer's calculated MPG is about 1.0-1.5 better that what I work out by hand (im getting 24.84 MPG on the freeway and 18.97 MPG "in the city"). Compare this with my 1994 ES 300 (20.84 city and 26.47 highway) and i'm still not afraid to take out the V8 over the V6 w/ gas prices the way the are (yet).
I use trip B to track my miles between oil changes.