vordmeister - 6:50 pm on Mar 24, 2011 (gmt 0)
I could have written the opening post - I'm in exactly the same position: [webmasterworld.com...]
I'm not convinced that site theming is causing me a problem, more that the forum on my site shows up in serps instead of the shop on shop related queries. My shop is getting 3% of the sites search engine referrals.
Is your shop software halfway search engine friendly? I couldn't find any that were any decent and ended up building my own.