incrediBILL - 11:20 pm on Dec 14, 2011 (gmt 0)
Google has been insistent that web hosts serve up a true 404 server response code when a page not found error is generated from an invalid url.
Google isn't THAT insistent or Google wouldn't be putting so much effort into detecting soft 404s in the first place.
Heck, I detect soft 404s when link checking, it's something that's so rampant if you don't do it the link checkers used by most are quite ineffective these days.
BTW, domain parks are the worst offenders, they don't want anyone to know the site has been moved, repurposed, and stuffed full of ads so all page requests are 200 OK.