After further consideration, I would say that maemo 6 not being on the n900 would be a bad thing only if:

1. maemo 6 apps can not be run on maemo 5. Since both will be using QT this seems unlikely. Also, the term 'Backports' comes to mind.

2. Nokia decides to focus on developing maemo 6 rather than giving us our portrait mode, better virtual keyboard. etc. Given Nokia's treatment of Symbian, Hanlon's razor may apply here.