A bit of googling and it seems that Comcast frequently has issues with their DNS servers.

Comcast has ocassionally messed up its DNS servers - last year they had a series of systemwide DNS outages that lasted several days. (My Comcast DNS servers are 68.80.0.12 and 68.80.0.5.) Today Comcast DNS is being slow - but not out. That is good, because it means I can still get to Google and I can still use it to find other DNS servers.

[ davidbau.com: Memorable DNS IPs ]

Based on this guys comment about OpenDNS, I switched to using that. I haven't a clue what OpenDNS is, or why they do it, but I can look into that after my deadline...