It's part of the problem with being early to market with this stuff. Most of the vehicles updated since the Model S came out that have connectivity packages include hotspot functionality. It's likely those owners are paying for connectivity (I haven't checked), but it is possible. My Model S gets better reception than my phone (bigger antenna, I assume), so I'd have some interest. Plus I have an old (but cheap) AT&T plan that prevents tethering my phone, so it would be nice.
Someone investigated previously and found our chipset does include HSPA+ (which AT&T calls 4G and is admittedly fairly fast), but it doesn't appear to be enabled on our vehicles.