This is not meant to be an exhaustive test, however I tried running Pandora and Google Nav. going into work, streaming the audio to my car's BT radio. I was using a Motorola OEM Charger (the one with the plastic circular window with the "M" illuminated by a blue LED.
Phone is running ADW EX Launcher, and I've frozen the bloatware and set the CPU to conservative.
I noticed with the screen off, during 20min of the commute, the phone picked up 2% of battery charge (the actual amount means just about nothing, I've only just gotten the phone and configured it last night). For the second 20min, I left it on the visible moving map, and over that period lost 2% of charge.
Now then, for those experiencing their charger not keeping up, I think the phone is going to limit the charging rate of the battery, regardless of the amount of current being put out by the charger.
I might suggest you repeat my experiment, and do two additional things: toggle to CDMA only, and adjust the screen brightness from auto to something lower. Doing both might help. I might even do the same, and perhaps report back those results
This is not a real issue for me as I generally keep my eyes on the road and listen to the turn-by-turn directions with the screen off.
FWIW: I've replaced the Pico voice with SVOX Classic and the American Gracie voice, which is much more natural sounding.