While trawling through what was a largely news-free VoIP for Business conference yesterday, I happened upon the stand of Babble, a VoIP company which is roughly as longstanding as Skype itself.
Anyway, their latest offering is a kind of business partnership with, well, anyone who wants to set up their own SIP-based VoIP service. They provide the API, administration system and the wholesale telephony itself. You rebrand it and resell the telephony to your customers as your own VoIP network - you can even use the client for marketing purposes.
It only costs £250 to set up, plus £75 per month to maintain (obviously they take a cut of the revenues too). Not a bad idea, really. Cool marketing and a minor money-spinner to boot, depending on your business model.