VoIP provider Vonage has already cut a deal with EarthLink to resell WiFi, presumably to go along with some sort of WiFi phone service, but could they have something way crazier (and more costly) on tap? BusinessWeek speculates that Vonage might be getting into the cellular game by launching an MVNO (Mobile Virtual Network Operator). It's not hard to imagine them offering something similar to T-Mobile's HotSpot @Home service, which lets you use a WiFi-enabled cellphone to make VoIP calls over WiFi and and then seamlessly switch over to cellular networks whenever you're outside WiFi range. The problem is that launching an MVNO is no small undertaking. ESPN shuttered MobileESPN last year after failing to attract more than a handful of subscribers, and Amp'd and Helio have spent hundreds of millions of dollars to carve out a slice of what is an incredibly competitive wireless market (Apple was working on an MVNO to accompany the launch of the iPhone, but scrapped the whole thing once it became obvious how difficult it would be to gain marketshare). Could they do it? Maybe, but not without spending a TON of money, which is why it's more likely that they'll introduce a more modest Voice over WiFi service.