Whitey - 9:05 am on Jul 26, 2011 (gmt 0)
@Sgt_Kickaxe - More analytics insights are needed to narrow this down. Bounce rates , longtail , time on site , etc etc over the previous year.
Always lurking is the potential for Google to remove pages and sites that do not convert well by default - aka low quality. The trend you describe seems to reflect this - but users intent and other factors can play into the conversions a lot, as others are describing.