Another computer, another instance of the Emacs 22 snapshot heading towards 100% CPU usage when Semantic is active.
The solution I tried before makes the Synaptic package manager complain about the Semantic version whenever I install any package. This time, I followed advice from the Emacs wiki and applied a patch to semantic-idle.el in place, byte-compiled the file, and moved the new .elc file over the existing .elc file (in a different directory).
It’s working fine, and Synaptic isn’t complaining.