This Tile tracking system for Apple devices is quite an interesting concept. (http://mashable.com/2013/06/26/tile-tracker-device/).
Apparently when the battery expires after a year, then you have to throw it away and buy a new one. But what interests me is that if its battery can last a year with small coin cell sized battery, then I presume one will able to make a low power BT module that should be able to work for several years on a AA battery.
The Nordic RFduino (www.rfduino.com) boards will open up nice possibilities to interface all kinds of things remotely to Gadgeteer boards. Did anybody else here back their Kickstarter project? They should probably be shipping the rewards quite soon now, but I have not heard anything from them yet.