The 10.5.3 update adds a new preference (on the General tab) in iCal for scrolling the week view by Days or by Weeks. If you set this preference to Days, and then press the Option key when scrolling, you'll scroll by week instead.

[robg adds: This was one of the first things I tested in 10.5.3 when I found the new preference, but I couldn't make it work, as I didn't actually change the setting in preferences. It seems (on two machines here, at least) that this shortcut only works in one direction: you must have the preference set to scroll by days, and the Option key then overrides that setting to scroll by weeks. If you have the preference set to scroll by weeks, however, the Option key doesn't then force scrolling by days.]