Blizzcon 2010 is hypothetical, for Blizzard has not yet announced such a show, much less given any details about when or where it might be held. It is widely expected that such an event will take place, after the wildly-successful and popular shows in 2008 and 2009, but nothing guarantees it.
The first Blizzcon was held in the Anaheim Convention Center in late 2005. There was no Blizzcon in 2006, before the show returned in 2007, and continued in 2008 and 2009. Since 2008 the show has grown fantastically popular, selling out in minutes. That said, nothing guarantees that the event will return in 2010.
Blizzard employees have made numerous joking remarks about "Blizzcon next year" when referring to additional info releases about their games. It's widely expected that the fifth character in Diablo 3 will be revealed at Blizzcon 2010.