The AP had a story yesterday about California considering a text message tax:

Text messages have gone from a major cost driver to virtually non-existent on enterprise bills. Texts get bundled with plans and given away for free. Of course, we’re texting more than ever. 

The AP story describes the hypothetical tax as a flat fee (most likely). If so, it would look more like an E911 charge that is common in most jurisdictions – which solves that pesky problem of getting 10% of the text part of your plan that is zero dollars.