I'm also having this issue. Any text I've written in the description column won't be found in any type of search.
Luke: What types of items are you adding descriptions to in the Project panel? I just tried this and noticed that filtering works only on footage items like clips and not sequences. Here I have a black video clip with "Kitty" in the Description column, a subtitle item with "Doggie", and a sequence with "Doggie" also:
If I filter on "kitty", the black video item appears:
Now I'll filter on "doggie" and only the subtitle item appears; the sequence does not: