Well, erm, from the release notes for the most recent update:
Pages 4.0.1 improves reliability when working with EndNote X2 or MathType 6, or deleting Pages files.
Ah, right, then. You can delete Pages files now. What?! You couldn't before? How did that work?
And, yep, that is actually the problem. The technical explanation appears to be that permissions of 'sub-files' (Apple has long had a fetish for using files which are not actually files, but either directories or magical filesystem meta-data things) get messed up when you transfer them to network drives. Oops...