What addresses did you try? You can try it other way:
http://btcaddr.me/[bitcoin address here]
Maybe the form is not working in your browser - which one are you using?
Firefox 16.0.2 on windows 7
-edit-
I first entered your site by your link provided in OP, then used the input field with a random address obtained on this forum (1CoinLabF5Avpp5kor41ngn7prTFMMHFVc). Then I changed some letters and later the whole address but it still gave me the same identicon.
Now I just tried it only adding the address in the URL, like you advised and it workeed. But dont you think it is problematic that the same address gets a different icon everytime I run it? Heck, it does the same thing again now, after Im leaving the tap open for 2 minutes, it returns the same identicon no matter what address I put in the URL.