Samizdata - 3:25 pm on May 23, 2011 (gmt 0)
Anytime you block a legit Googlebot - you face consequences.
Every time you restrict Google activity, a fairy dies.
Unfortunate phrasing, Brett - the whole point about the Google Web Preview bot is that it is not GoogleBot (which for PR purposes alone needs to maintain its reputation for honouring robots.txt instructions).
Google would not want your site in serps if they can't have a preview of it.
Google certainly does not want webmasters opting out of previews (hence the tactic of removing your text snippet if you follow their opt-out instructions).
But (so far, at least) they seem tolerate opting out by other means.
next-to-impossible to turn off
On one site I have been serving a 403 to the Google Web Preview bot for six months.
I do so because I consider making quasi-photographic images of every page on the site and using them to enhance a profitable advertising business is not fair use of my copyright.
The site in question consistently shows an empty (blank) preview and still has the customary text snippet. Ranking appears unaffected, as far as I can tell.
I appreciate that this could change on a Google-whim.
I also appreciate that most webmasters would not want to risk Google's petulance, and that many others happily accept visual previews as a fact of business life and would rather put their efforts into optimising how the previews look.
I think incrediBILL is right to highlight the NOSNIPPET issue, which might reasonably be described as coercive, bullying, or (in this context) "being evil". I would paraphrase Google's attitude thus:
"If you don't eat your meat, you can't have any pudding".
And if you don't eat your previews, you can't have any snippets.