In this case ESPN360 only makes its video content available to selected ISPs (including Adelphia and Verizon) but not to others (such as Cox, Comcast, Time Warner, and SBC). ESPN has regularly behaved similarly with respect to cable companies.
Proposed network neutrality regulations have had nothing to say about the inability of users to obtain content because content providers block their ISPs, or surcharges on ISPs by content providers for their users to have access to premium content. And this is even though there are often real monopolies on content (only a single provider owns it, and may completely control who has access to it, at least until it gets out to P2P networks), while there aren't any real monopolies on Internet access (though some network neutrality advocates have endorsed nationalization of "backbone," which would create a government monopoly).
I think that in general, the ISP does have more overall power and influence than the content provider, but there are exceptional cases where content providers like ESPN360 may have a stronger hand against ISPs. Overall, there's a lot more money spent on communications than there is on content (as Andrew Odlyzko's 2001 "Content is Not King" essay explained), and the real drivers of that spending are business and peer-to-peer communications, not content providers.