Thanks for the replies
The reason I/my company need the paper is for non-replicable certificates, so although the securtiy level would not need to be as high as passports or money etc, I'd like it to throw up problems whenever it was stuck in a copier/scanner.
First thought was holograms, which you can order, but powers that be are suggesting we look for alternative routes before spending out on it (*sigh*). dment's suggestion is pretty much bang on what I'm after- one certificate I'm looking at used very fine vector lines and dots that would completely shag up any scanner, and was thinking whether this was something I could try (providing I found a great printer), or just to try and outsource more.
'Tis tricky, that's for sure. I'll let you guys know if I come up with anything (unless I'm wrongly locked up on suspicion of counterfeiting).