I agree @Emasher, I don't see why people find it so hard to just say so. It's not like you're killing their puppy or something, you're just being polite.
I spent the weekend in Wales for my girlfriend's mum's birthday. Luckily I'd already met everyone that was there so there were no nerves about meeting people. I had a great time and get on with them all really well... good times!
...also, you know you're loved up when you change your Facebook profile pic to one with the mrs (yep, I have).