Thought I'd be all spiffy and upgrade to the shiniest new Wordpress tonight. I did my little svn switch/bzr add/bzr merge trick to get all the new files in the right place, upgraded the database, and felt pretty awesome... Then my blog started throwing miles upon miles of "transport errors" and 500s, as did a couple others that I hadn't even touched with upgrades. The common factor seemed to be WP instances that were nested off of subdirectories of one of my domains, rather than WP instances at the root level of the domain. That smells like some kind of configuration problem in Webfaction panel-land, but I was too impatient to open a ticket (and trying to focus on something, anything other than Mama Mia that my wife was watching), so I ended up doing a mysql dump, blew away my old blog instance, dropped a new one into place (which magically worked, thanks a lot, universe), and restored from my backup.
So... Upgrade done (good) but not how I planned it (bad).
On the plus side, I was kind of getting sick of the theme anyway. :-P