If you activate Mission Control whilst hovering the mouse over an app in the dock, it will do an Exposé for that app.
Activating Mission Control any other time will do a global Exposé as you say.
While in mission control, if you slide one finger up on the magic mouse (imagine it's a 2 two finger swipe with a trackpad) whilst hovering over the app windows, it'll Exposé the selected app's windows
In one of the earlier developer previews you could slide one finger up (on the magic mouse) over a dock icon while in Mission Control and it would trigger a per-app Exposé. Alas, it seems this feature has been removed.
There could be a hidden switch to turn it back on, but I've not had a play around with those 'secrets' apps to have a look yet.