As I pilot I've been around these for a while and due to my only recent ice fishing interest I never made the connection...until now.
http://www.sportys.com/acb/showdetl.cfm?DID=19&Product_ID=919&CATID=178We've all heard horror stories of CO hazards even deaths - since many of us spend good amounts of time locked in a shanty with a propane heater and/or lantern going it seems like a cheap easy safety feature. Even with the popularity of the buddy heater and it's low O2 cutoff...
I just picked up a few of them today (they are available locally at Tamarack and who knows where else) for $6 a piece - these last up to 90 days after you open them. They also have one that last 12 months - it's around $9 I think. (because they're usefull life is shortened by moisture and direct sunlight I went with the shorter life units...plus I figure two should get me through the ice season and then I won't need it anyway) I've got a $60 electronic CO alarm in the house and this is a cheap somewhat low-tech alternative for those other locations where it could be an issue - heck get one for your car while your there.
Haven't used it in the shanty yet, but I am guessing I'll be cracking windows or doors a lot more often once I do...just a hunch