I tend to agree that the S90 does not call for a really high performance memory card simply because you can't shoot very fast with it. I would choose based on reliability and durability, since this little camera will probably see some rough treatment.
Perhaps someone who has used the movie function can comment on the memory card requirement for shooting video.
Mike Brown
Vancouver, WA
S90, 5D, 40D, 350D, A720
My current Canons: 5D, S90, 7D, 550D.
Current lenses: Canon 28 1.8, 50 1.8, 85 1.8, 100 2.0, Sigma 50 1.4, Tamron 28-80 2.8, Tamron 75-200