The press release is now online, as is Mike Brown's Sedna page, at http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/ and http://www.gps.caltech.edu/~mbrown/sedna/ respectively. Right then - Sedna isn't a planet - it's not even a Kuiper Belt Object. According to Mike Brown, the Kuiper Belt has a fairly sharp edge at 50AU (1 Astronomical Unit is the distance from Earth to the Sun, 150 Gm); Sedna comes no closer than 70AU. The appropriate term for Sedna is Inner Oort Cloud Object
(and I'd like to take this opportunity to lay claim to the acronym IOCO
).
The Oort Cloud was theorised by a bloke named Oort, hence the name. It's a cloud (duh) of small icy bodies which inhabits the outer reaches of the solar system; Oort inferred the cloud's existence to explain the origins of dirty snowballs (comets). The Oort cloud is thought to lie much further out than this; it seems there is also an Inner Oort Cloud, apparently created when a passing star... passed.
Mike Brown also gives a very interesting suggestion of how to define a planet. He says a planet is any body whose mass is greater than the total of all other masses in similar orbits. By this definition, Pluto isn't a planet... but we knew that already.