I’ve hit that screen so many times in regular usage at home, at work, at a friends’, and at school. There’s no single thing that’s more likely to make me finally switch from Google to an alternative (probably Live Search) than all those *#%(#@% false positives.
I haven’t switched yet, and to be fair it’s been a long while since I’ve seen them. There was a time though where I was getting this at least twice a month and lasting for most of the day when it hit (and yes, ethereal and other network monitor tools confirmed that I wasn’t infected and spamming Google).