HubSpot just announced a paid public beta of a native version of one-to-many SMS campaigns for U.S. markets. While the product isn't as fully formed as some 3rd party integrations, it's promising that a native HubSpot solution is actively in beta and out of the "theoretical" space.
So what's available (and not) in this Beta?
While the service is a little pricey ($75/month for the first 1,000 messages + $15/month for a 1,000-message add-on). Third-party services are generally in the $7 - $10 price range for 1,000 messages.
You'll need to go through a registration process (which they note can take a few weeks) and be assigned a number before you can begin using the public paid beta.
We're looking forward to seeing more in this area as HubSpot continues to develop the product.