If you ceate a forum footer listing your company and services and post loads of discussions, is this considered SPAM (i.e. get penalised), does it make a difference, good or bad. Is it worth doing?

A footer such as:

Company Name (linked)

Service 1 | Service 2 | Service 3 (all linked)

Thanks

The footer is what appears at the bottom of each page displayed in the forum. It sounds to me like you are asking about a signature, which appears at the bottom of each message.

Whether the search engines regard it as spam is irrelevant. What matters is whether the forum operators deem it to be spammy.

I have found many SEO forum operators don't like links in their signatures. I, on the other hand, allow them on my own forums, and in fact I encourage people to embed links in their signatures.

We remove all obviously spammy posts, where people are just trying to pepper the forums with their links.

It's unfortunate that we have to police the system, but forum signatures are a long-standing practice which, in my opinion, is not likely to go away.

Nothing is spam if it's good for the users of the web page.

If a forum footer lists the forum owners other sites, it's good marketing.

I think asking whether it is "spammy" is asking the wrong question - the question to ask is whether you'll see any benefits from it.

From a search engine perspective, Google already has methods of devaluing links from the same /24 range of IPs at least, let alone a single domain. And so far as I can tell, Yahoo! isn't isn't overwhelmed by links. So you may sdll find the search engine benefits of such links very limited.

So you then have to evaluate whether human users might possibly find those links useful, or whether you are likely to put users off from joining or posting, if such links do nothing but make your forum look like an amateur link bomb.