Can I include SimplePie with software I'm selling?

Short Answer: Yes

Long Answer: SimplePie 1.0 is licensed under the BSD license where the only restrictions are:

1. Redistributions must include a copy of the license (which includes the copyright notice).
2. You can't use the names of SimplePie or the SimplePie team to promote your product without explicit permission.
3. Regardless of what happens, we are not responsible for anything that happens as a result of your use of SimplePie.

So yes, you can use SimplePie in a commercial project. No worries. :)