The Daily Republican Register
has a nice write-up about the difficulties in accessing 911 from VoIP. In short you need to make sure you find a service that offers E911 and I would definitely test it shortly after getting the service (not all services have 911 available immediately after hookup).
Check out their article
Though the discussion of SPIT (SPam over Internet Telephony) was not a major topic during the VON Expo, it was brought up. Jeffrey Citron, chief executive of Vonage, answered a few questions in regards to security issues but he apparently wasn't ready to share their strategy.