I feel that to understand the reasoning behind the Symbian purchase by Nokia one must first understand the technology that the company holds.
From visiting the Symbian site, I see compelling technology that is key to Nokia's business strategy.
The following technologies that Symbian owns have caught my eye.
1) SMP
2)LBS
3)Freeway
4)Screenplay
So as I see it, with the Symbian acquisition not only does Nokia get improved software technologies to make their phone more battery efficient but also technologies to further advance the phone's capabilities.
Final Thoughts,
It seems to me that all of these companies are acquiring technologies to further advance mobile phones, and thus add more feature enhanced capabilities to them.
IMHO, .mobi is unaffected by these changes and eventually .mobi will benefit from it, once all companies begin implementation of these advancements, since mobile standards will be revised to the new standards.
In reality I believe we are seeing the beginning of a transition from basic xhtml mobile code to xhtml + enhanced mobile code (mobile optimized sites with more video, picture, better GUI, etc). Just like the changes websites went through from regular html with links to xhtml/ajax/css + video websites we have today!
Nokia started .Mobi, and are the one backer that has really gotten behind the extension. They already owned about half of Symbian, they now have bought out the other partners. So, whatever their grand plan is, it seems likely that .Mobi is an important part of it.
From what DBADT2007 has posted, Symbian is a powerhouse. This is very encouraging, in an indirect way.
Given your explanation, I would agree that mobi is not affected in a detrimental way by this purchase.
Nokia has been a stalwart partner for mobi, so perhaps they have a plan in mind.
It would be nice if the mobi partners would give the extension a needed shot in the arm once in awhile, by at least mentioning the extension in their press releases.