Thanks for the hint.
I used the SCM to stop the indexing service, but it would not respond to the stop request - so it seems indeed to be stuck. While waiting for it to stop, I looked through the search filters (with SearchFilterView), but didn't find anything out of the ordinary. I ended up killing the service process, and restarting the service, and now Classic Shell is returning results again.
One thing you could do is adding a "Waiting for Windows Search Service ..." instead of "Searching..." if you don't get any response after a few seconds.
Since it's not your fault, you might as well put the spotlight on the service that's responsible for hanging - plus it'll give a clue to anyone on what's to blame.
