Amazon Prime customers with iPads can rejoicethey wont have to buy a Kindle Fire in order to enjoy the company's Instant Video service on a tablet.

The Amazon Instant Video app for iPad launched Wednesday morning, and acts as a sort of hybrid between Netflix and the iPads native iTunes video store. Like Netflix, customers of the $79-a-year Amazon Prime service have a library of videos that they can access at any time; and like iTunes, the service also offers a vast store of a la carte movies and TV showsup to 120,000 videos in all, and many of them new, fresh-from-the-theater releasesthat can be instantly streamed or downloaded for later offline viewing. But there is a catch.

Amazons Kindle app for iOS long ago removed all store links rather than give Apple a 30 percent cut of in-app purchases; likewise, you cant buy or rent new videos or subscribe to the Prime service directly from the new Amazon Instant Video appyoull have to have to conduct such transactions directly through your browser, either on the tablet or on your desktop computer, then shift over to the app for viewing. Thats an added step that makes the movie-buying process somewhat less seamless than using the iPads native iTunes store. (If youre not already an Amazon Prime customer, the app provides a number of visual hints enticing you to behow else to watch Downton Abbey?but without telling you specifically how.)

The app has several other features: Watchlist lets users queue up individual movies or whole TV seasons that they wish to view; the list is available in the cloud, so you can add a movie on your iPad and then check and adjust the list when you move to your computer.

And if you do own or have rented a video from the service, it shows up in the Library section of the app. From there, you can watch a trailer for the movie, choose to watch now over wifi, download for later offline viewing, or add to your Watchlist.

The launch of the Amazon Instant Video app comes during a week of increasing video availability for Apples customers; on Tuesday, Hulu Plus was added as an option for Apple TV. Wednesdays news signals, too, that even though the Kindle Fire provided one of the first legitimate challengers to the iPad in the tablet market, Apples lead is still big enough that Amazon doesnt want to forgo that sizable customer base by limiting its video service to its own device.

The app is free, and compatible with any iPad running iOS 4.3 or later.