Found from one of my Google newsalerts:
“Taiwanese hardware and motherboard supplier Asustek Computer is to launch a wireless Ethernet-accessed hard-drive in May.
Its WL-HDD will offer fast WiFi access by using 802.11g, which provides up to 54Mbit/s. The actual drive will be a 2.5-inch unit and its capacity is as yet unspecified. The device will cost $150 about £90. It will have a Web-based management interface through which the drive can be accessed. Files will be freely shareable, have read-access only or be restricted to password-owning users.”
Imaging building these into buildings, spaces, walls: geocaching dead-drops for digital media. I remember Matt Locke talking about something like this a couple of years ago – a city-wide network of storage where the city’s inhabitants would exchange news, media, anything digital. Mass-storage ‘hotspots’. I left my Pixies in Maida Vale…
» Macworld Daily News: March 23, 2004: Wi-fi hard-drive unveiled
Petafloptimism 
There have been a few of these before (Sony released a Vaio-branded one in Japan, and there’s the Martian NetDrive (which does iTunes/daap sharing), but I don’t think either of these were based on 2.5″ drives, and they certainly weren’t that cheap.
I don’t think they’ve (yet) made much of an inroad into the market. For them to become invisible will be a long, tricky process, and I’m not sure how it’ll happen. Geeks probably won’t drive it, as they have existing home servers in a lot of cases (2lmc certainly does), and the public is only just “getting” wireless, let alone the idea of personal servers.