Shortly after I made the switch from Drupal 5 to WordPress 2, I started thinking about the various apps and hosting providers I've used to run my blog. I actually had to mine the archives to remember it all, because it's changed a lot. Over the last 5 (or 6, depending on how it's measured) years, I've used 6 different applications, on 4 different hosting providers. That speaks volumes about interoperability, making it easy enough to move to a completely different weblog applications on 5 separate occasions. Sure, there was some data altered and URLs adjusted, but all posts and comments made the transition successfully each time.
I've now been on WordPress 2 for just over a month, and have no plans to change weblog applications again (although I've said that 5 times before...) I may still be changing hosting providers, if Dreamhost doesn't work out (it's going pretty well, and the company is really nice to work with, but they've been having a LOT of issues lately. Cats in ur serverz or something...)