iOS: Amazon just updated its iPhone and iPad apps in a big way: You can now send documents to your iOS device via a new Send-to-Kindle email address, including PDFs. There's a new layout too for viewing books, newsstand, and docs.
iPad users have some additional upgrades, including access to over 400 magazines and newspapers and "print replica textbooks" that include the formatting and layout of print books as well as the ability to highlight and otherwise annotate those books as expected. You can also rent some textbooks from the iPad Kindle app. These features -- and the PDF support (most titles in Amazon's PDF format, Kindle Print Replica, are academic) -- are a big boon for students, naturally.
The apps also support Kindle Cloud, Amazon's 5GB of free online storage. So documents your email to your Send-to-Email address are automatically archived online and re-downloadable on other devices.
You can grab the update here:
Kindle [iTunes App Store]