It will read 255 (or whatever the max value for 10-bit resolution is) until the voltage falls below 3.3V.
The internal clamping diodes will protect the pin as long as I supply enough current limiting to it. Is this right?
That is my understanding. It is also my understanding that protection is obtianed by the diode conducting. Which would be a constant drain on the battery just like the voltage divider (until the battery drops below 3.3V). Which brings us full circle to my earlier question. Is the voltage divider that much of a drain to be a problem?
I don't really know. It's a 400mAh battery. I hate the idea of it just draining down until the battery protection circuits kick in even when the device isn't being used.