Surprised no one has started a topic on this, so here goes!
Personally I think it could be a good move, providing the existing (DVD-ROM) customer base is addressed carefully. I would pay a small supplement to upgrade to the Steam version.
The possible downside to Steam distribution is that FSX is a highly customisable title and a "Restore Cache" could play havoc with personal installs, such as FScene terrain updates etc.
Also FSX is becoming very temperamental on modern hardware, maybe less so with Nvidia than ATI cards, so DTG could have their work cut out as regards tech support unless they can hammer some updates into the code base.
Certainly seems our favourite train sim company is branching out into wider transport simulation, which can only be a good thing for our hobby.