From an email from Marco:
I’m happy to report that I no longer test Instapaper.com in IE6. I actually don’t know if I ever did, but I sure don’t now: since the site’s core functionality requires a bookmarklet installation, and IE makes bookmarklet installation so clunky and difficult, I don’t have the time to walk IE users through it. And recently I added AJAX features and rounded corners that, while they degrade to acceptable functionality, they definitely don’t work in IE (6 or 7), and I’m unwilling to spend any time to hack together equivalent functionality that’s IE-compatible. As an individual doing this in my free time, I can’t afford IE compatibility.
Instapaper’s IE users, therefore, just need to accept an inferior experience. Sorry, but it’s either that or no new Instapaper feature development at all. –Marco Arment