Plugin or method needed for Twitter style 'load more' endlessly scrolling page

Hello,

I have a blog which at the moment uses standard pagination with links for "newer entries" and "older entries" to load in newer or older posts.

This needs to be updated so that older posts load via a "load more" button, similar to seen on Twitter. So that the posts only load when specifically requested by the user. Even better would be for the posts to load automatically when the user scrolls to the bottom of the page.

Are there any WPMU plugins which would help with this? Or do you know of any other plugins or methods which could be useful?

Any help would be greatly appreciated.

Thanks,
Shaun