I've been following this topic over on Ars for a bit, and it's one of the few cases where I actually bother to read the comments. I have no background in network stuff, so please correct me where I'm wrong:
Comcast and L3 have a peering agreement, which applies to traffic that is coming from one network and moving through the other network. That is, if Comcast wants to send something to a third party and the easiest route is through L3's network, the peering agreement applies. Per the agreement, no money exchanges hands as long as the traffic sent through each network remains about equal. For data intended for a network, there is no charge.
Netflix previously was hosted through Akamai, who paid Comcast a fee to gain special access to Comcast's network. As Akamai is a straight-up CDN, they had no real network of their own -- or at least no back-bone -- so Comcast was essentially their ISP. Netflix is now contracted with L3, so traffic enters the Internet on L3's network and goes where ever it needs to go.
Now, L3 is acting as a CDN in some capacity because they are hosting Netflix's data, and they are also acting as a service provider because they are providing the connection to this hosting service. Comcast sees that L3 is now sending a lot more traffic its way and wants L3 to pay extra due to the increased bandwidth usage. Further, L3 has requested some number of extra ports so that they can send this data to Comcast further, and Comcast has given them a small number of the requested ports but is balking on the rest due to costs. They are asking L3 to pay for the additional ports, and to pay an additional monthly fee for each port.
I can see this a couple of ways. Say Netflix hosted with AT&T now, and they sent their traffic through L3 on its way to Comcast. In that situation, why would L3 be liable for extra fees to Comcast? On the other hand, if L3 is asking for additional equipment so that they and/or Comcast are more easily able to handle the load, then yes it may make some sense for L3 to foot some or all of the cost of the hardware upgrades they want Comcast to put in place. I guess my real question is whether Comcast is asking for money simply for profit's sake, or are they asking L3 to pay for the improvements to their network that they are making for L3's (and tbh, theirs as well) benefit?