I think it doesn't really matter. Nowadays a phone lives 1 year before it is outdated...maybe 2. In a year from now the telecom provider will make you a deal with a new phone that won't be a N900 but its successor. I will buy the MeeGo phone.
I think...if you can make Qt apps for the N900 then you can for its successor.
Although...there is a slight feeling of betrayal the overall feeling is that this will be interesting times.
If you can make great Qt mobile apps on the N900 you will be king of the hill on MeeGo.
Until MeeGo is out that is what I will trying.

Regards,

Theo