Took the plunge and upgraded to Lucid today. Here are some notes on what I had to do:

• Standard dist-upgrade worked fine, no issues with drivers.
• Thunderbird was upgraded to version 3. My QuickText plugin stopped working, but I was able to install it again from Add-ons, which worked despite complaining that it wouldn't.
• QT Phonon is still broken. This is the bug, and none of the workarounds work for me, unfortunately. But this is not a regression; it was broken in Karmic too.
• Mozilla Sunbird was uninstalled for some reason. Went and got another copy, which installed fine and found all my data, but the site says it's the last Sunbird release, and we should move to Thunderbird + Lightning. All very well, but the default Lightning plug-in is not 64-bit. Had to find a 64-bit download here. Installed that in Thunderbird, and then exported my calendar from Sunbird as ICS and imported it into Lightning, without problem.
• Lots of 3rd-party sources were disabled; re-enabled them, switching "karmic" or "jaunty" to "lucid" in the paths, and did updates, getting a new VirtualBox in the process.

If any other oddities appear, I'll document them here.