Apparently the folks at Earthlink missed one of the essential lessons imparted in kindergarten, namely that sharing is good. Simultaneous access covers hardware solutions like routers or Apple's own Airport or software solutions like IP NetRouter or SurfDoubler. Here's a good analogy.
Same thing applies for Internet service.
I would consider my Internet provider, PacBell, with whom I have a DSL connection. I did a quick check with them to see what their policy is on connection sharing. I quote from their FAQ page . Our technical support center does not have the expertise and cannot assist you with networking. While not exactly helpful, it certainly isn't prohibitive.