While my advice is generally consistent (if you need it now, buy it now) there are great resources for tracking whether or not you should hold off -- especially when it comes to Apple products. Certainly keeping up with sites such as MacRumors.com will give you a pulse on what the rumor mill is saying (with varying degrees of inaccuracy). A section of the same website -- the Buyer's Guide -- tracks time between previous updates and contains helpful recommendations such as: "Don't Buy - Updates soon" (current the advice for Macboook and Macbook Pro systems).
While this information fits squarely in the "individual results may vary" category and you may not be able to wait for the elusive and unconfirmed updates 'soon' -- there are occasionally times when waiting just makes sense.
Apple has invited press to an event on October 14th. According to Engadget, this one's all about notebooks. So, ya... it'd be a bad idea to pick up a new Macbook this weekend.
Unless of course, you must.