Crashing for me, too. I've deleted and reinstalled to no avail. This is after upgrading from an iPhone 3G.
It was clear with the 3G that when I left the app, it wasn't getting enough time/RAM to "clean up and update the stream because when I came back I'd have to redownload the same messages I had just read. I was looking forward to the 4s, thinking it would work better. Silly me.