There's Beagle and Gumstix that I can think of immediately that were designed for a job like that specifically. Both run Linux, and Python, and have been around for a few years now.
And the Arduino has a 'Ethernet Shield', which is a plug-in card to handle the physical connection so you can stream data over its IP address as well as control its ports. Granted this needs programming in C+, but the hard work is done for you with its community libraries and example code.
Pi's are great, I've got one. But its a bit trendy, and only uptake in education will tell if its more than that. Arduino is more capable, but is a pig to program for. However enough kids learning python might tip the balance yet in Pi's favour, although it doesnt have a lot of hobby IO potential. Its all in the graphics on that one.