There are many different things that can affect your CPM. eg. the industry you work in, the economy, time of year, etc. It's hard to draw too many conclusions, I think, unless you're getting large volumes of traffic that show very distinct trends.
keywords could be a reason.. my cpm dropped by 50% within 1 day.. and now its been like that for a week. advertisers backing out because of the slowing economy could be a reason.