Amazon has updated its Kindle for iPad app to support background processes, allowing users to download ebooks without leaving the Kindle app active. The new version, v2.5, takes advantage of iOS 4.x's multitasking support, as well as the ability to side-load content received as Mail attachments or loaded via iTunes.
In addition, there's "improved" image zoom while reading. Kindle for iPad is a free download, and can be used to buy books from Amazon's ebook store or to read free titles. It automatically syncs content with other Kindle apps or devices linked to the same user account, including the point up to which you've read in an ebook.