Of course, as Maemo and Moblin are both Linux based they'll already share the vast majority of their architecture already, so merging Maemo and Moblin to produce MeeGo is a case of choosing which bits to keep and which bits to drop (ok, huge simplification!) and then moving forwards with the new combined Linux distribution.

But fundamentally...

A "MeeGo" OS that is DEB based is Maemo.
A "MeeGo" OS that is RPM based is definitely not Maemo.

The next major release of Maemo (aka MeeGo) may well include some new features or functionality that derive from Moblin/MeeGo but ultimately it the next release has to be DEB based otherwise N900 owners will see much reduced app support as it's not reasonable to expect developers to support DEB and RPM packaging.

What happens once a version of MeeGo appears with RPM only support is then anyone's guess - presumably the next device (ie. first MeeGo device) will be upgradeable from Maemo6/DEB to a version of MeeGo with RPM support.

Obviously the N900/M5/DEB will eventually be out in the cold, unless the community can come up with a port of MeeGo/RPM which may be more likely due to the reduced level of closed components in MeeGo.

The best option all round would be to move as quickly from DEB to RPM and try to avoid a situation where both packaging systems need to be supported concurrently by developers.