They heard it. You can purchase the $10 Unlimited International Text add-on (CDMA only), which allows you to text as much as you want from the U.S. (including from Canada, U.S. Virgin Islands and Puerto Rico) to over 180 countries. Note that it requires an unlimited domestic message plan on the subscription before the service can be added. I'm checking if having the $20 Unlimited SMS plan also meets the requirement.
I confirmed that any rate plan or add-on that contains the SMS feature will satisfy the requirement. There are a few exceptions like the 300 & 1000 metered message options. So if unsure whether your plan or add-on meets the SMS requirement, Customer Care will verify it for you.
You can most certainly text to Mexico as much as you want with the $10 Unlimited International Text Add-on as long as you're in the U.S. or a domestic location, including Canada, US Virgin Islands and Puerto Rico. See here for the list of supported countries: http://support.sprint.com/support/article/Country_codes_for_international_text_messages/case-ba416758-20090619-165829?&INTNAV=SU:SP:MVT