I've seen this objection from a number of people now, and I'm curious: what sorts of careers would be destroyed by someone finding out you played WoW? I'm not doubting that such a career exists, I'm just having trouble coming up with an example personally.
Any career where the employee represents the seriousness and trustworthiness of the employer. Sales being one, law being another. For people in these fields, everything they post with their real names should be about their field, on well-known and well-respected blogs and review sites. If an employer _knows_ that their clients will Google representatives, they cannot hire someone who does not have a squeaky-clean and "serious" online image.
Heck, I was _almost_ found out because I had an article covering statistical analysis published in a online webmag. Was I proud at the time? Of course. But the webmag was about a hobby that is usually not considered "serious" or "professional." Good thing it was noticed after I was hired. In the competitive market I am in it could have sunk my chances.
And that was a published article with solid math and favorable feedback. Just think of standard PvP trashtalk or guild recruitment posts. Not a chance.
Most employers want work-a-holics. They say they don't, because eventually their productivity suffers. But they do. For example, most employers, if they could get away with it, would discriminate against someone that was married and has children. Single guys won't complain about dental coverage, can stay later more often, won't be distracted because of XYZ home issue, whatever.
If an employer finds out that you have a hobby that you do for many hours each day, that is scheduled in such a way that it might interfere with working late, that _might_ occupy your time with non-work issues, well.... what do you think they would do? Unless they really want YOU and ONLY YOU, they will move to the next guy. They can't pry into your personal life in an interview - it's against the law. But they can google you.