Nokia divides their devices into four categories: Explore, Achieve, Live and Connect. Before they only had two "letter" series for the smartphones, Nseries for the Explore category and Eseries for the Achieve category. Recently they have trademarked also Xseries and Cseries which belong to the Live and Connect categories respectively. Examples of the phones in these categories could be Xpressmusic phones(Live) and 6000-series(Connect).

Cseries will be for the basic affordable entry level smartphones so I don't think we'll see MeeGo under that. The impression I have is that its intended category is Explore and I'd be surprised if we see MeeGo on anything else than Nseries in the (nearish) future.