Yeah, you don't want to make it look like spam! Put some 'teasers' on the page like you said. How else do you plan to tease it if your not really teasing? If you don't want to give away your idea, try to be as vague as possible with the content on the site.

How about making a survey to see if you can see if users are interested in your product/service and the same time your offering some idea of what your product is (or so they think). This is a great way to kill two birds with one stone.

Tip: Change up the survey every so often to see if you're still consistently getting similar responses.