Fortunately, it's easy enough to work around (the nice thing about developing a Facebook app is that the layer of indirection means that I can redirect the port at the FB layer without any change at the user level), but it continues to increase my desire to quit this annoying company. Do I understand correctly from recent conversations that RCN (for a small surcharge) allows inbound port 80? That alone might get me to sign up for them for Internet, given how incompetent Comcast has been lately. (Have I mentioned that outbound email through Comcast has been consistently failing for us for the past two days?)
For now, I seem to be back up and running. I'm tentatively assuming that they simply noticed my inbound port 80 traffic and chose to shut it down. (Although, in that case, I have no idea why they were allowing it previously.) If I find that my new port gets blocked as well, it means that they're sniffing my traffic and looking for HTTP, in which case I'm simply out of here -- we're paying them a small fortune per month, and if they want our money to go elsewhere that much, we can probably oblige them...