From the Amazon Software Update page. It looks like they are adding some things that we have been asking for for a while:
Kindle Version 2.5 Overview
The 2.5 software update for Kindle and Kindle DX is coming soon. We are rolling out the new software update to a limited group of Kindle users and plan a broad release in late May 2010. Check back here for updates on the release schedule.
Here’s a list of the coming enhancements:
Collections: Organize your books and documents into one or more collections.
PDF Pan and Zoom: Zoom into PDFs and pan around to easily view small print and detailed tables or graphics.
Password Protection: Password protect your Kindle when you’re not using it.
More Fonts & Improved Clarity: Enjoy two new larger font sizes and sharper fonts for an even more comfortable reading experience.
Facebook & Twitter Posts: Share book passages with friends on Facebook and Twitter directly from your Kindle.
Popular Highlights: See what the Kindle community thinks are the most interesting passages in the books you’re reading.