Frankly, 10 years ago, multi-monitor support was a *much* better idea than it is today.  The economics of cheap, very large flatscreen monitors vs CRTs has changed the dynamics, such that virtually everyone goes for the largest single monitor they can afford (which is now hovering around 24" for the typical gameplayer).  That is, the gaming population tends to upgrade their main monitor size, rather than add a second monitor to their existing setup.    Back around 2000, I knew a bunch of gamers who had (2) 17" or 19" monitors, and the rich ones could splurge for (2) 21" (or a 21" and a 17").  Now, virtually everyone I know has a 27" or a 30" LCD/LED panel, and nothing else.  The only dual/triple screen folks I know of are graphics/video people, and a few hard-core coders.  And only the later group games seriously, and they're tiny in numbers.

Now, there's still a good case for the effort into movable screen UI components, because, even on a single screen, that can lead to significantly better playability (and be a real selling point attraction).  And some of that effort will allow for multi-monitor support.  But pretty much any effort that goes into explicitly supporting 2 monitors (say, the ability to view both the main galaxy view and the planet view at the same time) isn't going to be worth the developer time, as the portion of the target audience which has this as a selling point is smaller than ever now days.

I also have 3 monitors at 1920x1200 each and was just thinking a multi-monitor game would be very innovative. I think once the graphs are complete it would be useful to monitor the overall economy at the same time as the tactical display around individual ships. If we had a window based approach, we might be able to monitor interesting areas of space around each ship at the same time, rather than switch back and forth constantly. Other windows could be fore planetary governance, etc.  

I would also like to see the map/mini-map be zoomable so I can see the whole area of active space at once.

That may be your anecdotal experience, but I personally know many gamers including myself that run multiple screens. Now, I personally don't usually run game across multiple screens, using my 2nd for web pages, music, etc. but it is still an extremely common set-up.

I too have two monitors  (1280x1024, 1366x768) & being able to plop a largerversion of those right side menus onto the secondary monitor would be great.  Having it as a floating window might actually be beneficial because it might make it easier to handle varying resolutions between monitors & it could potentially be more open to allow greater extensibility down the road if it were a floating winforms/wpf thing you could just have spawn  on a different monitor to drag to others