NotebookReview wrote a report on how the use of WiFi affects your battery life. Here's a snip from their conclusion:
Simply turning on the wireless card doesn't drain much power, it might shorten battery life by 2%, so if you want to remain connected to download email it's no big deal.
If you turn on wireless and are constantly trying to acquire a connection, such as if you're constantly dropping a connection and then trying to reconnect, your battery life could be 6% - 7% shorter than usual due to the wireless card having to work extra hard.
If you have wireless card on and are transferring lots of data, such as downloading large files or constantly browsing the web, battery life could be about 6% shorter than simply no wireless on. Consider downloading large files later when you're plugged in.
If you don't need to use the web, turn wireless off, it'll save you at least a few short (but maybe precious) minutes of battery life!