Some ISPs have started shipping new DSL routers with (*gasp*) all incoming traffic closed (so no one can originate a connection back in to your network). I praise them on this, personally... though your issue doesn't sound like a simple block, are you sure you're not running something else that's answering the connection and then dropping it? Blank pages are normally the signs of something like TCP wrappers (or similiar) or a proxy...

Also, you are on a routeable network, right? Not RFC1918 address space?