Historically Google updates backlinks about 4 times a year, or every three months. However, it's been longer than that this time -- probably because their resources are tied up with preparing the new Big Daddy infrastructure. In his blog, Matt Cutts mentioned that he wasn't sure when backlinks wouold get updated, but after Big Daddy was a good guess. This pushes it to March at least.
However, if you've been getting new backlinks, Google DOES register them continually and use them in calculating your real PR and your rankings on various searches. It's just that the information isn't exposed publicly with the link: operator or through toolbar PR. And the link: operator will never show you anything more than a sampling of the backlinks that they actually see, and that sampling is usually the less important ones, in my experience.