Yes, there are C++ APIs for SMS applications on Series60 phones.
You can find lots of tools, SDKs, and general information for
Series 60 development at
"www.forum.nokia.com" --> "Series 60 Platform"
Here, you can find the document
"Designing C++ Applications for Series 60"
More documentation can be found under
"View all Series 60 documents >>"
From the Series 60 page, go to "View other Series 60 downloads >>"
Here, you can find helpful downloads like the "SendAs v1.0 Example Application for the Series 60 Platform"... which has example source code used for SMS and e-mail functionality.