I am going to drive my M3 sparingly once winter arrives. (I'll take it out when the roads are dry to avoid salt is my current plan) I figure I should get a battery tender to be on the safe side. I did some searching and it looks like I want a BMW Advanced Battery Charging System.
Here it is on Amazon for $70:
http://www.amazon.com/BMW-Advanced-B.../dp/B0016BRA3M
The Amazon review says that "BMW deactivates the cigarette lighter on recent BMW models."
After searching the forums I found: "All e9x vehicles are equipped with an Intelligent Battery Sensor (IBS) on the negative terminal which turns off (as commanded by the DME) within 1-2 hours after engine shutdown.
If you do use the trickle charger, make sure it is directly on the battery terminals (in the trunk) rather than underneath the hood."