Err, bummer - first thing I'd do is swear.
On a beetle you can get to some of them from the inside of the car by just taking the trim card out, so you might be able to get the fallen nut out with a magnet and then just fit a standard nut on the end of that bolt.
VWDCQ vice pres & web dude - vwdcqinc.org.au
Join a car club - get car peeps
My toys
'70 Hillclimb Bug 'Jezebel' w/- Haltech EFI -
http://bit.ly/VwTNtQ
'60 KG Coupe - resto kinda started
01 New Beetle - gf's car, but I turn the spanners