What causes battery to drain faster than it can charge? I have a Moto DROID (v1).  On a trip from Tyler, TX to DFW airport I had my phone in the car dock and charging.  I was using Navigation for directions to the airport and also had Listen running to play a podcast over the device speaker.  As I neared the airport, the phone rebooted a few times and then remained off.  I took the phone out of the dock and noticed it was very hot to the touch.  At the airport, the phone had cooled and I plugged it into a wall charger.  The battery was completely drained!  

My brother had the same experience with his Moto DROID (v2).  He was on a 4 hour road trip and was using the navigation app and participating in a conference call (hands-free).  The phone was connected to the car dock and charging the entire time, but after a couple of hours the phone rebooted and then shut-off with a dead battery.  We left the phone off and plugged into the charger for a while and it was able to boot again, and like with mine showed a dead battery!

Is the phone really able to drain the battery faster than it can be charged via the car dock?  If so, are there known combinations of apps or uses that will cause this behavior?
I think you answered your own question ;)

Navigation uses a lot of power: it needs constant use of GPS along with reading the map and processing text to speech.  Throw in another power-intensive activity like bluetooth or the phone's speaker (or calling, or data, etc), and it's very conceivable that it would drain power faster than it can pull it from a charger, especially a car charger, though the exact specifics would vary from phone to phone.

Here is a great list of the things that use the most power on a phone.