Google just announced at IO that they are adding the ability to rent through the Android Market. The Market added Books not too long ago, and Movies looks to follow in the same way.
Users will be able to rent movies for 30 days through the Market and stream to your device. If you don't want to stream the movies, a playback option called 'pinning' will allow you to watch when you have no connectivity. Any movie you rent will be available on all your devices and the web to watch instantly.
There will be Movies app for tablets starting with Android 3.1 and phones with 2.2 in 'a few weeks'.
Source: Android Market