You could look for days and be hard pressed to find a Wi-Fi device quite this silly: Nigel Ballard alerted me to this Target.com item, the K-Byte Zipit Wireless Instant Messenger, for $99.99. It's as ridiculous as its name. It lets you instant message with Yahoo, MSN, and AOL services if you have accounts on those IM systems. (This isn't mentioned on Target's site, but is noted in an Amazon.com customer review.)
The device works with Wi-Fi networks--it must lack any kind of stub browser because it refers to using just home and free networks, which means that if you have WEP or WPA security enabled or use a gateway page, you must be out of luck. The product details say there's security for instant messaging, but doesn't describe it. I'm guessing it means password protection, not encryption. (A reader suggests its application-layer encryption for instant messaging systems that support it.)
It is worth noting that Motorola has been selling a similar device for around $60 for quite a while now. (The Motorola one does not use standard wifi, however, nor does it support anything other than AOL.) It may seem ridiculous, but I don't think it is meant for anyone sophisticated enough to read a blog like this, nor for mobile use. The target market of these things seems to be families with more kids than computers -- if you've got an IM-using kid in the house, this may be a cheap way to end arguments over who gets to use the PC all day. Kid gets to IM from the privacy of his or her room or from in front of the TV, mom and dad get their computer back, for a lot less than it would cost to add another computer.
It might replace two-way pagers or carrying a laptop when all that's needed is IM for mobile workers, but I kind of doubt it, unless they have a VNC client that works on it.
As for the kid IMing in the privacy of his room, I've wondered with Motorola IMFree (and this?) since the device works through the family PC, are Mom&Dad able to monitor the IM stream?