Its not as wide-spread as many people are paranoid that it is, but it certaintly goes on.
I've had a couple of attempts on my WLAN. I know with certainty that these attempts had to been from wardriving assholes for a number of reasons. One: My WLAN is 802.11a and does not spill off my property. My property is too large. With just a standard client, someone would have to drive all the way up my driveway to get even a very weak signal. The attempts had to of been done by someone with a directional antenna with quite a bit of gain. Two: In addition to the 802.11a for my WLAN, I also have 802.11g and 802.11b. The use the 802.11g for media stuff and it is secure, but the 802.11b (completetly seperate network) is open. It is an internet-only pipe that I don't mind anyone using. (I am using a captive portal for content filtering and logging though.) I have a couple outside repeaters for the 802.11b. If these attempts were by someone just wanting to get an internet-pipe, they would've used the 802.11b and not made any attempts on the 802.11a. Three: The time that the attempts occured; late Friday and Saturday nights. Wardrivers haven't yet discovered females, so they spend their Friday and Saturday nights doing so-called "research" and trying to "save the world from itself". If they could get a girlfriend, they would probably lose interest in people's stupid little $100 plastic boxes.
Cheers, Eric