Very cool, simple implementation of pubsub with webhooks. The DoS issue is something to consider, but keep in mind there are plenty of other ways to DoS, probably more efficient ways. Auth is a big topic in webhooks right now.

The allow file idea is interesting, but we've been talking about it on the webhooks list (as well as the GetPingd list). The pubsubhubbub guys are also thinking about it (since they haven't addressed it either yet).