They finally found it - again - another tenth planet, this time unnamed but temporarily designated 2003 UB313. It was also announced yesterday that 2003 EL61 may be roughly 70% Pluto's size.
UB313 however, is definitely larger than Pluto, although perhaps only slightly. According to the Washington Post, Mike Brown has labelled the object the tenth planet; they acknowledge that there are astronomers who disagree with this label, but don't mention that Mike is actually one of them. As I wrote about last year, he doesn't consider Pluto to be a planet, and argues that if Pluto is considered a planet, many other objects (including Sedna) must logically also be considered planets.
So UB313 must logically be classified as a planet as long as Pluto is. I'd be willing to bet that by this time next year there'll be another object found to be larger than Pluto, if not several; then, the International Astronomical Union will have to reconsider their definition of what constitutes a planet (or come up with one). For now, if you're asked how many planets there are in the solar system, it's definitely wrong to say
Discover Magazine discusses the outer solar system in more depth in their November 2004 issue; you'll have to log in to read past the first page of the article, which usually entails using BugMeNot.