Yep, I've got ideas. The same thing happened to me.
Basically, a couple things are happening. First, your own machine doesn't need a proxy to filter through, so it views your IP address directly. Second, your ISP (DSL, Cable, Satellite, whatever) may be blocking port 80 (the default port for serving web pages, of course).
My machine allows me to type in my IP address fine; but then, for internal HTTP requests, I had to route port 80 through port 8000. Then, the person requesting your web site must type in your external IP address (not your internal IP address if you are networking) and then :8000, ie:
http://128.282.319.202:8000/
Also, don't forget this is the main webroot. If you want a particular user's page, it's probably
http://128.282.319.202:8000/~someuser