Gabriel García Márquez wrote: "Everyone has three lives: a public life, a private life, and a secret life."
Sometime these lives intersect on the Internet. And this is why people are encouraged to be watchful about what they do on the Internet. The advice is to not engage in anything that might come back to haunt you because Google, in its zeal to index everything it can find, will find things that you might not want to be found.
But is this really true? I did an experiment and Googled "Tom Foremski." Google found 135,000 references. I started clicking to see if I could find the very last search result, number 135,000. But I couldn't, I could only get as far as 552 results before I got this message:
In order to show you the most relevant results, we have omitted some entries very similar to the 552 already displayed. If you like, you can repeat the search with the omitted results included.
I clicked on the "omitted results" and got even fewer results, just 196 links. Either way, the most pages viewable that were indexed "Tom Foremski" was 552 -- just 0.4 percent of the total index.
So it's not true that an employer, or anyone else, can easily find potentially embarassing things about you through a simple search.
While being careful about what you post is still good advice, I have even better advice: be very active on the Internet -- anything bad will be diluted into the long tail of you.