Nothing’s actually changing with the way Google customizes search results, it’s just going to start making it clear what’s at work.
Starting today, you’ll start seeing little messages at the top right corner of your results. They’ll look like this:
When you click on “More Details” you’ll get this message:
Which basically describes the three areas that might receive customization, namely: location, recent searches, and web history. For the full explanation, head to the Official Google Blog.
Now at least, when your SEO clients see their web site listed at #1–when the rest of the world doesn’t–you’ll be able to point them to an official explanation! Or not, depending on your level of evilness.