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 nine
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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.