LOL! If I only had a $1 for everytime that's happened to me Colin! ;-)
Get a telescoping magnetic pickup tool at local auto parts and you can fish them out from under the engine with it. May take a few tries but I've always managed to retrieve whatever nut, bolt, wrench or socket I've dropped while working on mine. If you leave them in there they may well be rusted by the time you next find them and it sucks having to go buy new tools to replace what you dropped...
Good luck!
- Michael