There is information and discussion of this topic elsewhere.
Ari Jaaksi says, "So what’s with Maemo6? Maemo6 will be MeeGo compatible.....consider Maemo6 already a MeeGo instance."
Quim Gil gives a more explanation on talk.maemo.org:
Harmattan is the name of the software program that comes after Fremantle / Maemo 5. In the Maemo Summit we already announced it as "Maemo 6" in order to bring a clear signal about the major update, change of toolkit, etc.
Now MeeGo comes to the picture. "Maemo 6" will not be used by Nokia as a brand since all the marketing effort around the software platform will be around MeeGo.
To be clear: this is not about "ditching" or "abandoning" any platform. The Harmattan program keeps working with the same plans than last week, no matter the name of the product they will deliver. Maemo 6 and Moblin 2.x merge and have a successor called MeeGo. Current Maemo people will look at it and will say "looks like his mother!". Current Moblin people will look at it and will say "looks like his father!" (or choose your preferred gender) Of course you will see changes compared to Maemo 5, but these changes were coming anyway with Maemo 6.