Personally, I think Dave missed the boat. Two key points: CAPEX and OPEX.
For CAPEX (capital expenditure), there are ITSP's that are offering a leasing package which includes the phones, a router, a POE switch, Internet Access, phone service and monitoring. It is financed and rolled into one monthly charge, eliminating the CAPEX scare.
There are a number of businesses that just need the IP desk phones that they opt to purchase themselves. At $125 per phone that isn't that big of an expense compared to the proprietary phones of Avaya or MITEL.
VoIP is sold as a cost savings over TDM, especially over traditional ILEC TDM offerings and MSO bundles of Internet-Phone. Yet, the Hosted PBX model is more about Total Cost of Ownership and Business Productivity particularly today with our tech heavy needs in business and our increasingly mobile human resources.
An example: I was at the Volvo dealership having my car serviced today. Their tech guy had to disconnect the free wi-fi in the waiting area because the TDM T1 went down. But Bright House cable was still up. The dealership couldn't operate its business without Internet Access and a savvy Tech person. (The Internet is managed by one of ADP's divisions).
The other key point was that MITEL does indeed offer a hosted PBX service (based on the 3600 system by Convergence and Proximiti). And I believe that Avaya either has one or bought one through the Nortel purchase.
We have a mobile work force today. Everything is going Cloud. You need access to the Phone system, email, apps, data, CRM, etc. Not much of that is premise based anymore. (It is being pushed to the Cloud by many, many companies in the Web 2.0 / Business 2.0 space).
And if it was premised based, who would keep it updated, secure, backed up, etc.?
Final note: Unless the business is buying an Interactive Intelligence PBX or the top-of-the-line six-figure PBX from the Big 3 (MITEL, Avaya, Shore-Tel), the company isn't getting all the functionality and features available via most Hosted PBX models.
UPDATE: I edited the mistakes from my hurried post this morning. ADDING: There are folks that do Hosted PBX poorly, but there are certainly companies that do hosted PBX well. The BC/DR aspect of a hosted PBX solution adds a benefit to the mix that premise-based can't duplicate.