From the release notes:
Instapaper now supports the “ihttp://” URL scheme to add pages. (Thanks for the idea, GoodReader.) If you can’t get the bookmarklet, the email address, or Copy working to add pages for some reason, or you just don’t like any of them (hey, you never know), there’s now another way to add pages to Instapaper from the iPhone or iPad: Edit Safari’s URL field and insert an “i” before the “http://”, tap Go, and Instapaper will launch and offer to add that page.
Awesome. Shame on you if you’re not using Instapaper.