Im new to all this Arduino network/ethernet stuff as well..
but wouldnt the Arduino need an Ethernet shield then? in any fashion? to be able to 'connect' to this web service/site that is remotely hosted?
On the opposite side.. (and Windows based only) which is a bit different than your scenario as you have/want your web servers t be in another location form the Arduino..
but I have installed WAMP server locally on a PC.....
created a FLASH based GUI/interface that allows me to send RGB values to a PHP script.. that in turn dumps the data to a com port that Arduino is listening to....
works great normally anywhere inside the LAN.. if I wanted to open it up the public.. I only need ot log-into my router...and forward any HTTP traffic to the machine with the WAMP (web) server running control the Arduino from the 'net'..