WordPress MU

OK there is something very odd going on.

I've got blog posts that I can see in the Database

BUT I cannot access them via their permalinks, nor do they appear on the Edit posts list. Well they DO if I start using filters - but the edit posts page only shows about 20 posts even though I have 40 and seems to have no way of paging through them

But if I access them via the old ?p=post_id permalink then suddenly their peramlinks start working again.

I vote plugin interference.

Well so do I - but I'm trying to work out what it could be ;-)

Does anyone else get a smaller number of posts shown on the Edit posts table than there are in the DB and not get any way of navigating to the other post?