I emailed comcast sports net and got the following reply
-------- Original Message --------
Subject: RE: Why did you block the game on Verizon Fios
Date: Fri, 19 Sep 2008 10:02:54 -0400
From: Askcsn <email@example.com>
The black out of last night's Phillies game was a technical error on part of Verizon. We are unsure of the details but you can contact Verizon for more information. Thank you for your interst in Comcast SportsNet.
Sent: Thursday, September 18, 2008 9:58 PM
Subject: Why did you block the game on Verizon Fios
Why did you block the Phillies game on Verizon Fios?
When I called Verizon tech support, I was told that Comcast blacked out the game due to low ticket sales. Fine black it out for the home team, but not the visiting team. I never heard of such a thing in baseball.