Do forums pass on the same PR and SERP juice as other relevant sites in a sector. I am noticing more forum threads in the serps these days - just wondering if this is a cost effective way of link building?
It is well a cost effective way of building the reputation of your site and getting traffic, but it doesn't carry much wait if any with the SE's
Getting a (good) reputation = good public relations and therefore will get you links that have weight with the search engines. Good old publicity also works wonders for search-engines, for the simple reasons that people will blog about you and link to you. This is a road that isn't available to everybody, but I have seen it work. Forums in your niche work the same way, but in a way that is available to most webmasters (those who know how to write and behave themselves). Businesses too can tell an office-worker who types fast and knows the business well to devote one hour a week (or something) on posting in forums - that will certainly help in the link-building department: indirectly. Whether the signature-links mean something for the SERPS (directly), I don't know. Would depend on the popularity of the forum.