Wouldn't be hard to see if it were the shock leaking. But, a leak seems obvious if you've had a collection of oil form.
I've had mine for a little over 10,000kms, since new (June 2006?); it lusts for twisty roads and smoothly eats up the fast country miles, I haven't baby'd mine through it's short life and there's been no issues.
But, I have found issues (on o/s forums) with Nexus after work has been performed: do your brakes work OK? I saw an issue where the rear hydraulic line wasn't re-secured after wheel removal and it got in the way of (whatever?) and got scuffed and eventually caused a hole/cut... That's what I did before I got mine: checked out all the forums and checked out what, if any, were typical issues and the main one i found was slack/sloppy mechanics who didn't re-secure things or check tightness on things...and this issue still persists I've found. You pay for care-factor in your labor fee, I wish some of them realised that; must say my recent service at MyScooter got top marks.
Removing the paneling is a pain, they don't lack nuts & bolts, they're everywhere!! Your mech should trace the leak once he gets inside the covers, could be a tension issue on the engine housing/busted gasket... they don't have quality issues on the later versions but did have some on the earlier ones (series1?)
Getting parts from Italy is easy, getting them through the importers here in Aus is the problem, they seem to take forever from what I've heard of; I wanted a red-trim panel for the steering head on mine, 'not available', so I got a friend in the Nexus Club to send me one... 4days. lovely.
Learn Italian and get onto the Italian Nexus Club site, they have many Nexus and heaps of info!! very helpful site!
Good luck with yours, keep us informed. Is yours a Red one... you know the Red ones are faster!!! 
