wireless temperature sensor

Not wifi but a standalone XBee + temperature sensor will do this for around $30-$40 or so. The XBee has a uC + A/D converter and could read an analog sensor. At a 0.1% duty cycle (transmitting for 1 second every 1000 seconds) you can get around two years on two AA's with a fairly simple design.

I have a short ZigBee writeup at http://wiblocks.luciani.org/white-papers/intro-to-zigbee.html

One solution would be to use a a JeeNode and a JeeLink (plugged into a PC or router). Cheap, open-source and programmable from the Arduino IDE.

Another vote for a JeeNode radio and receiver. I suggest you review this weather station wireless implementation:

JeeNode Weather Station