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As someone who doesn't use IE, I'm more impressed by a site saying "This site is only compatible with IE right now - we're working on it, though!" than with a site that is obviously broken and incompatible. And if you don't intend to ever make it work in the other browsers, say so up front.

And users will figure out very quickly that it only works in IE; better to be up-front about it.

Let them know it works only in IE and be honest about your plans for getting it to work in other browsers.

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).