According to the Maemo marketing team, Nokia will move its entire N-series range to the Linux-based Maemo operating system by 2012. With the Nokia N900, Nokia started the transition at the high-end of its device range. Over the next few years, it will gradually push Maemo down to the mass market. By 2012, all N-series devices will be powered by Maemo.
In the meantime, Nokia still has a number of Symbian-based N-Series devices in development. We can look forward to at least a few more Symbian-powered N-series devices along with some new Maemo-powered ones over the next couple of years.
At this point in time, Nokia does not expect to transition its E- and X-series to Maemo but the option has not been completely ruled out either.