I just switched hosts and almost ALL of the old blogs are now on version 2.3.1 of WordPress. If you haven’t updated yet then that’s just naughty.
The best thing you can so is always keep up to date with the most recent WordPress version. It keeps you hacker proof (most of the time) and will keep you up to date with the recent advancements with arguably the best blogging platform in the world (insert evil laugh here).