I have the same problem. It just started happening - I've been playing it for a few weeks.
I had Angry Birds for about 5 months and it started freezing about 3 weks ago. At the time I was almost maxed out on my harddrive space as I had a few movies on the NC (and I don't have a SD card). Once I removed the moview and now have 4 of 5 GB free Angry Birds is not freezing anymore.
P.S. - After I updated to 1.4 yesterday there was an update for Angry Birds