The other thing i personally found was irrelevant links punished me
I had a page rank or 4 (not great but still ok) and every forum i used i put links on, and after about 2 months my traffic increased but my rank dropped down.
After speaking to a friend who works for a standards and seo company he tells me that google looks at the context of the linking site, so if a plumbing site is linking to a porn site, its not likely to be genuine is it?
He also stated that a car site linking to a model site would ordinarily be ignored if not negative but if their is some acompanying context/text with the link then most search spiders will take it in context...
Also if said car site has other links to similar sites then that helps...
So he tells me on forums, bbs, social and business networks, to include some well chosen keywords and then write a paragraph of text containing readable sentances but also keyword dense with a few links in there, and make sure the links have alt tags, ID and name tags and are compliant code and use that to link in sigantures as it can convert strange but relavent link into an acceptable link...
So be careful what exactly you link from...
Its also worth noting as well that spamming be it comment spam, forum spam, newsletter spam, posting spam or any kind, does have an effect... ensure that all mass postings include a 100% definate removal process 'click here to unsubscribe' with a generic link to ur site that then asks them to enter their email and stuff is bad. but a phrase like 'This email was sent to
s@xy.com, if you wish to unsubscribe and not recieve our special offers on products, keywords, bumph, etc, then please click here and then comfirm it'. Now its essential for some search engines that the recipients email is in this lil block as some sites use BBC so the email it was sent to is hidden.
Now you may be asking why add keywords and why is that bit of text important its only an email... well gmail, yahoo, hotmail etc use the content of an email to deliver adverts and they also spider links from email, so use it wisely, and the option to opt out in there indluing the email, is what the engines like to see as the laws have changed/changing on spam so they like it to a standard.