<p>Do you think ASP.NET MVC will ever have a significant share of the Microsoft web development market? Or will it be more like 10-15% of the market?</p>

I think it will be a slow start because of the webforms install base.  As unit testing and markup control starts to become
more important to more people
then I think you start to see a migration.  I doubt that it will ever reach 100% of the MS install base for a web platform but it will be a steady rise over the next years to come.  There will be the fan-boys who will say this will dominate planet sliced bread but that just isn't realistic.  With that, I'm hoping not to have to use Webforms for a long, long time.
EDIT:
Now that we are at Beta 1 we are
starting
to see the component vendors start to ship some stuff.  Looks like first up to bat is Telerik with their
ASP.NET Ajax Controls in ASP.NET MVC