Some shows are there... but it all depends on who produced the series. It is not dependable to find these episodes available On Demand.
I have noticed, however, that some of the networks have started letting you sign up with your Dish email/login and get access to stream shows if you miss them.
My DVR screwed up this Monday, for example, and I can logon to FOX Web site and view episodes that aired on Monday this week instead of waiting until next week when they go live for everyone else... and they usually have several older episodes as well.
ABC and NBC even have iPad apps so I can watch their shows on my mobile device.