You can use mixed arrays with strings and JSX elements (see the docs here):
<MyComponent text={["This is ", <strong>not</strong>, "working."]} />
There’s a fiddle here that shows it working: http://jsfiddle.net/7s7dee6L/
Also, as a last resort, you always have the ability to insert raw HTML but be careful because that can open you up to a cross-site scripting (XSS) attack if aren’t sanitizing the property values.