Few hours into the Jaunty Update, I ran into the Tracker index corruption issue mentioned in the Ubuntu 9.04 Jaunty Release Notes.
In some cases it can happen that the index of the "tracker" desktop search engine becomes invalid. A dialog will be shown, offering you to "Reindex all contents". This button does not work, and the tracker service might start to use large amounts of CPU and disk resources. As a workaround, please press Alt+F2 and run tracker-processes -r. This will be fixed in a post-release update soon (361205).
However tracker-processes program was not installed on my laptop, so I installed tracker-utils package and ran the tracker-processes -r which seems to fix the problem.