The iTunes Store is a great place to find episodes of TV shows that you might have missed, or even catch up on past seasons, but it's only one of several options available.
Netflix offers a free iPad app to watch movies and TV shows for those who have signed up for their $8/month streaming service (or higher). An iPhone/iPod touch app is also available. Netflix still has more shows available via DVD than instant streaming, but there's still plenty there.
Hulu has a free Hulu Plus app for iPad or iPhone/iPod touch for those who use their $8/month service. Unfortunately some shows which are available at Hulu.com are not available to Hulu Plus users, but there is still a lot of content available. My biggest complaint with Hulu is that their content contains ads and their catalog changes, so something that is available today might not be viewable tomorrow.
The latter problem is due to content owners and their complete lack of understanding about how customers want to be able to access their content. I still don't understand why they are charging me to view the content, and then also showing me ads. Yes, I understand that cable TV has worked this way for years (I pay for a package which includes channels like A&E, but A&E still shows me ads), but Netflix doesn't show me ads, so why does Hulu?
If you want to watch your own TV on your iPad, you have several choices including EyeTV and SlingPlayer. EyeTV costs $5 for their universal app, while SlingPlayer charges a jaw-dropping are-you-frakking-kidding-me-with-this $30 for the iPad version and $30 for the iPhone version (sold separately). That doesn't include the cost of the equipment you'll also need to purchase, ranging from the $200 EyeTV HD DVR to $299 for the Slingbox PRO-HD. I think it's safe to say that the SlingPlayer executives who thought that up should expect some coal in their stockings this Christmas.
I've tested the EyeTV app and found its performance, even over a "Wireless-N" connection to be pretty weak, although I'm not sure if that was due to my older Mac hardware, or their software, or the limited amount of RAM in the iPad. I have not tested SlingPlayer because my hardware is not compatible with their ever-increasing specifications, and I'm not willing to shell out more money to reward their greed when there are so many other options available. Engadget has a walkthrough of SlingPlayer Mobile for iPad here.
There are many more options than the few I've mentioned here. If you have Dish Network, there's a free application to let you watch content while mobile. Sadly I have DirecTV which doesn't even have an iPad app to control my DVR, but if you subscribe to their NFL Sunday Ticket package, there is an app for that. ABC has a free iPad app to watch their shows. CBS has a 60 Minutes iPad app for $5, CBS News (free) and the free tv.com app.
For more information, you might want to read some of our previous posts on the topic of TV on the iPad. We've written about the EyeTV Hybrid and EyeTV app, Air Video and StreamToMe, and even about hacking the Apple TV to stream just about anything from the iPad to your TV.
Assuming that you have a Wi-Fi connection with sufficient speed, enough money to pay for the various services, and (in some cases) the proper equipment, your iPad can deliver all sorts of TV content to you.