Ok so I thought it was my new laptop acting up, possibly with keyboard/trackpad drivers. It didn't happen often and I ignored it. But now I've seen it happen in GLB on my old desktop which is a totally different setup to my laptop (32bit Vista AMD vs 64bit W7 i7).
Basically it shows it can paste something but nothing pastes when you select it. If you try and paste into *any* other application (eg. notepad) then it will paste correctly. Just GLB refuses to paste anything.
Restarting GLB has no effect. Logging off and in again has no effect. Restarting the PC fixes it.
Cheers