The best answer we have been able to give all of our clients who are concerned more about quality than search engine indexing is: you shouldn’t worry about the frequency as much as about the consistency as long as you are posting timely and relevant information. What we mean by that is, your site will set the expectation of your readership as long as you provide great content.
For everyone else, you need to post at least (2^PI)*4.56/sqrt(40) posts per week. Ok – so this works out to roughly three times a week, but since you are writing for a web crawler, does it really matter?
Update: We just noticed this article titled “How often should you write a blog post?” that strengthens our position stated above.