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The update boils down to one feature, albeit an interesting one: the app now has a new "stacked card" interface for selecting and managing multiple conversations. Before this latest update, one had to manage selected conversations and currently open conversations with two separate (and similar) toolbars. Now, when you select multiple conversations, you'll see them "stacked" on the right, and you can archive, trash or move them easily.
The new version of Gmail for iPad also features some sleek new CSS3 transitions, described in detail on Google's code blog.
To try the new Gmail for iPad, open gmail.com in your iPad's Safari web browser.