Yes they do and are a ggo start to getting your site noticed on the Internet. Do a backlink check on your site and see what comes up.
Of course do links in forum posts count. Of course do they have weight. Are they "devalued"? Why would they be? The only thing with forum links is that there are so many other links on a given page that those links don't carry much weight. You would have the exact same result if you overload your homepage with links.
Unique class c IP's linking to your site are a plus. A forum with sig links will have a ton of links. If a website exceeds a certain amount of outbound links, the search engines take notice and take away from there weight overall in the algo. I am not sure if it is specific to forums or just to sites that can be link farms in general. Having the links in the sig might get some click thrus to your page from users, which is way more valuable than having your link show up on a link farm page or forums.
Ya I agree link from DigitalPoint really counts I have seen lots of websites having a high PR and high rankings are getting links from DigitalPoint Forums. Akash Kumar
Unless you can provide proof that Google has an algorithm to detect all humanly possible forms of forum posts for the sole purpose of devaluating the links in them, I will stick with my belief that the only "devaluation" that occurs is because there are so many links on a given forum page. The exact same outcome of which will occur when you overload your standard website with links.
I completely agree with Icheb. My Travel Guides website runs a small little forum that is not using one of the "big" BB scripts. The urls are not suspicious in any way and except the main forum site do not include the word forum. They do not include any funky strings (&,?,= ...). There is no possible way, that Google can tell THIS forum post apart just from its url. They could analyze the page content, seing that it consists of multiple posts, but again many blogs would fit that criteria.
Signatures I doubt have any weight at all apart from directing search engines in your direction, but they couldn't have a negative affect otherwise it would be too easy to adversely affect competitors.
ofcourse they count as backlinks.. see the back links for my Cook It All site here. But they don't contribute much to PR. They have way too less importance than other natural links. Reason for their being less important is that if each page allows 20 posts to be displayed, and each sig has on average 3 links, you can easily have 60 links on one page, all for unrelated sites. Whats more, some people might link from within a post to some site(s), which can easily increase the link count on a page to 65 or 70 or more.
Have to agree with Rosiee. There is no way for google to identify all forums as such, especially those with clean url mods. Haven't thought about the number of links, but then again people happily submit to directories. Those also have tons of links.