Over at Daring Fireball John Gruber has discovered an interesting change in the recently released 10.5.3 update. There is now a checkbox in the Spaces tab of the Exposé & Spaces Preference Pane which changes what happens if you switch to an application (via ??? + Tab or the Dock) that does not have a window open in the the Space you are working in. With the checkbox checked it continues to work the way it always has, viz. it will switch to the Space with a window open for that application. If it is unchecked it will not switch spaces, but rather merely activate that application in the Space you're working in.
Gruber considers this is a significant change, because it allows you to use the Spaces on a task basis rather than an application basis. The behavior of the Dock icon also has changed. Clicking multiple times on the Dock icon of an application with no open windows will produce different effects: clicking once will activate the application in that space; clicking a second time will switch to a Space where it has open windows. Be sure to check out Gruber's full description for more details, or just try it for yourself.
I completely agree that this is the way Spaces should work, and it's great to see Apple providing the option for those of us who prefer to work this way. If you prefer organizing Spaces by application rather than task (the "classic" Spaces behavior), just leave the box checked.