goodroi - 9:39 am on Jul 4, 2012 (gmt 0)
For a site with over 45k pages you don't provide many details about what you could be doing to offend Google.
Did the Google communication mention unnatural linking?
Have you ever bought links?
Have you ever used hidden text or cloaked content?
Does your site use scraped or auto generated content?
How many unique words are in an average article?
Why do you think its the forum and not the article pages?