I've noticed more and more mentions (both in posts here and in actual job descriptions) of programmers's "portfolios" - typically their public profiles on sites such as this, github, etc.

How important is this, and would companies (startups in particular) reject (or immediately discard withouth even interviewing) otherwise outstanding candidates who don't have an online presence?

Personally, I prefer to keep a very low profile online. My name here cannot identify me, and I have other handles for other sites. I have a very spartan (and completely private) Facebook page. I do code on my own but the code lives in local repositories. In general, the less information online about me, the better.

I could see a designer needing some sort of online portfolio, but for a programmer, is this really a big negative when job-searching?