This could be a bug, but the idea in general is not to type a prefix of the file you want to find, but some characteristic letters. For job.model.php you could type jm or jmp or mod. For app_controller.rb try ac or acr or con or something like that.
Brilliant! Didn't realise the algorthim was that good (duh - I guess I should have when Allan said he donated to quicksilver). I wonder how many other things I am missing out on because of my diminished expectations...?