Earlier quoted context omitted.
Myself and some guys with a telecoms background spoke to various telcos about that ~2 years ago. Feedback was that they were already so over stretched with data usage they weren't going to embark on any projects likely to increase it until there had been a lot of network upgrades. May have changed since then though.
I think the biggest risk in doing this for a VPN service is that the usage is impracticable given its nature and secrecy. With services like Facebook, Twitter, etc, the telcos know more or less what they can expect in average data usage from users (and plan/charge accordingly).
Therefore, a VPN provider will be in the same position. So instead of paying AT&T for your data plan, you're basically paying the VPN people for your data plan plus privacy from AT&T.