Hopefully the next guy trying to debug the data logger shield will find this. The real time clock doesn't work on the Arduino Mega 2560. Apparently some pins are in a different place. RTC won't work on a Mega.
If you run the DS1307 sketch from the RTClib examples folder, as it says to do in the http://www.ladyada.net/make/logshield/rtc.html
IT WONT WORK. The serial monitor shows repeating time info, but the time doesn't change. Just the same as if there is no battery. And to make it more frustrating, it does not display the "RTC is NOT running!" message as the documentation shows.
Thankfully, it works perfectly well on an Uno, or I'd still be tearing my hair out.
Perhaps some admin could put a note on the page above? I spent an hour debugging this. I thought I fried something. That wasted time would have been avoided if the Adafruit pages noted this incompatibility. None of the Adafruit Data Logger pages mention it as far as I can tell, and that's not right.
Generally a *really* happy Adafruit customer.