win2win - 2:15 am on Nov 16, 2007 (gmt 0)

One advantage with a subdomain is that if, for some reason, the subdomain catches a penalty, it doesn't necessarily affect the root domain. With a subdirectory, it is much more likely for trouble to affect the entire domain.
That said, a domain's internal linking is a big part of how Google decides what pages are the most important. So if you stay with the current directory set-up, I would give some thought about how you offer links from the ecommerce directory to the forum.

For example, you could offer links to the forum with an iframe so that even though your visitors get a link from every page, there's still only one link. Then you could offer links from forum pages back to your ecommerce pages in the normal way. I think that linking structure would send a message about the relative importance.