Anectdotally, I have noticed that sites containing forums with the "normal" directory structure of site.com/forum rank better than those with a subdomain (forum.site.com).

From what I have gathered, G views these as two distinct sites.

Has anyone else noticed this tendency?

Interesting. I think awhile back subdomains were the thing, but recently I've read a few articles and posts that seem to suggest just what you observed. I've never really tested it myself to know though.

as an independant entity sub-domains have to generate enough link popularity of their own, sub-folders benefit from the link pop of the root site.

sub-folders will also benefit the main site by generating deep links

Chris said it all.
Don't use subdomains without a strong reason (like scalability).
If the forum and the website are on the same box, there is no need to use a subdomain.

All good to know. I had thought that was the case, but it's nice to have confirmation. Fortunately I've always gone the way of subdirectories.

Thanks for your comments. They have basically validated what I have seen anectdotally.

I would venture to guess, however, that the search engines will eventually, if not currently, give greater weight to sub-domains in the future (pass some degree of PR and link relevance to sub's). It's actually counter-intuitive that sub's would be penalized (albeit not intentionally) because they're a different way of structuring site content.

Just my two cents. Thaks again.

I agree about the possible future of subdomains. They seem like a logical way to structure a site, particularly as it grows past a certain size. I wouldn't think it would be too hard for a search engine to figure out it's all part of the same site either.

it is highly unlikely that sub-domains will ever be treated as part of the root host. the technology behind DNS is such that they are seperate entities and should be treated as such. A sub-domain does not even need to be on the same IP as the hostname is, just as long as the DNS servers for hostname have the subs set as A (HOST) Records not CNAME (ALIAS) it will resolve correctly.
Technically and quite correctly www is a sub-domain of hostname.tld and we are all aware of the canonical issues that exist with Google caused mainly by their means of trying to merge the www and the non-www versions of any site.
It is purely through common use that www has become the accepted norm for website addressing

There are thousands of websites using sub-domains of uk.com all owned and operated by disparate companies. What would happen if suddenly they were all treated as one big site ? I sure you wouldn't be happy if you were one of those owners.

I'm afraid you are looking at things the wrong way around.

The search engines have to work with the technology NOT the technology having to work around the search engines.

That makes a lot of sense Chris. I hadn't thought of it that way at all. I guess too that if you get to a point where your site is large enough to make subdomains a good choice in structuring your site then you're probably alreayd generating a lot of traffic and aren't going to need to be as concerned with any seo benefit or lack of benefit from the subdomain.

For most of us a directory structure for our sites is all we need.