WordPress 3.3 was just released a couple days ago. And while it’s a great release and there were tons of improvements, other things do break along the way. It’s impossible to account for 100% of every variation, of every plugin, in every theme.
That said, the WordPress core team does a mighty fine job of trying their best and I feel it’s more than adaquate.
In the 8 years I’ve been using WordPress, I’m only had my theme break once (and that was very early on, in the days when “tags” and “pages” didn’t exist.)
As with any major WordPress release, there is typically a X.X.1 release to follow within a couple weeks.
Ipstenu, one of the forum Elves, started a thread describing the existing known issues so far. So you may want to check that list before you upgrade.