I'm not on the Trello team, but I've spoken to them about this, and the short version is that it's tough as nails to do this in a sane way that also works across a wide-enough cross-section of browsers to be worth the dev time at the moment. As far as I grok things, that's part of why they're focusing on native apps. I'll hope someone actually on the team, like hamidpalo, responds with more details.