if i mentioned the url of my blog in a forum thread say for example http://xxxxxxx.blogspot.com. is it considered 1 backlink?
Yes, if that forum allows signature links and doesn't auto insert the ref="nofollow" tag. They tend to have little to no SEO weight due to the fact most of the time that page is not related to your websites content.
yes but usually of very very very low value, as there will be many other links from forum sig files. It may help to get your page indexed faster to give the link in a popular forum such as this, but it will not much for your PR at all or much for serps.... just how i see things.
True. ANY link pointing to your site helps. Just some backlinks are worth more than others like Adulu says!