Here's a couple of interesting videos, obviously very specific to the particular apps:
"Developing Erlang at Yahoo" http://bayfp.blip.tv/file/1281369/ - Talks about the transition of Delicious from Perl to Erlang, including live migrations of the DB.
"Taking Large-Scale Applications Offline" http://www.youtube.com/watch?v=cePFlJ8sGj4 - in particular, the section on "Versioning and phased rollouts: what to do when you can't assume all servers have the same code"
"Developing Erlang at Yahoo" http://bayfp.blip.tv/file/1281369/ - Talks about the transition of Delicious from Perl to Erlang, including live migrations of the DB.
"Taking Large-Scale Applications Offline" http://www.youtube.com/watch?v=cePFlJ8sGj4 - in particular, the section on "Versioning and phased rollouts: what to do when you can't assume all servers have the same code"