I experience exactly the same thing. When opening bitmaps in Illustrator CS5, they look jagged. The zoom is at 100%. The bitmaps can be anything, eg JPG with 300 ppi resolution.
I’m pulling out my hair…. It seems to print OK from Illustrator, but if you save a new PDF from Illustrator containing the bitmap, it looks exactly as in AI, bad resolution.
I never had this problem in CS2! I can’t stand it how much Adobe charges for their software, but then they get simple things like this wrong….