Jeeze, sorry guys... that was pretty stupid, at least I'm not accidentally grafting in rules from 4th Ed yet. Right, so Monstrous Creatures are just like other models for the sake of cover, and I couldn't find the bit where it says they cant go to ground either. This is fine by me because I have a Tyranid army.
As for cover and forests... the rules seems pretty clear in the book, but for 'realism' reasons, a few people might say, 'but there's only a 1mm of forest between me and the other guy!'. So if anyone thinks that's likely to happen, maybe saying something like:
"if your in the forest and more than half of the models (which can damage the target) are tracing their line of site through less than 2" of the forest, you don't give a cover save to the target".
... again though, this is more of a house rule because I cant see anything that contradicts the whole, if the line of sight passes through a forest for any reason, target gets a cover save. I might speak to my local group of friends about this and see what they think for our games.