Definitely let users know, and block access from other browsers. It's better to see an "Sorry, IE-only for now, please come back later" message than a broken we

Definitely let users know, and block access from other browsers. It's better to see an "Sorry, IE-only for now, please come back later" message than a broken web app. Personally, I can't imagine starting up IE just to check out a new web app, unless it seems insanely cool... so if you're looking at not alienating FF users, it's a good idea to get it working in FF asap (Firebug makes JS coding sooo nice).