I went to both Twiliocon and interviewed with Twilio. They're very smart guys, but the problem is that its a race to the bottom. Their idea is innovative (webapps <-> telco integration), but anyone can do it once they learn how to handle the SMS messages between the web and the carrier gateways, as well as how to handle call flows with Asterisk or Freeswitch.
Also, to me, it appears they're swimming upstream. Everyone else is moving towards unlimited voice/sms, while they're trying to stick to per minute/per sms rates (even for their VoIP offering!).
Awesome idea, excellent execution, not a long term business model.