It depends on your definition of “secure.” You control where the links go and how they are distributed. The private streaming table will show the you the number of visits and number of unique visits for each link created to help you understand if something may have gone farther than you wanted.
We proactively monitor the unique visits for any stream, and will notify you via e-mail when a link receives more than 100 unique visits in a 48 hour period. For any compromised link, you can easily cancel that link by selecting the trash icon next to it.
…But a stream is a stream, and if music is coming out of a computer’s speakers, any 8-year-old can capture that music with Soundflower and have it up on MegaUpload RapidShare before their bus gets home. If that doesn’t sit well with you, we recommend you keep your music off the internet until its release date, period. If, however, your goal is to let press people stream some tracks from your new album while restricting the general public’s access, and you’re at peace with the risks that entails, then private streaming is for you.