Actually this route is a lot easier sounding than a hide-away plasma mechanism.
Interesting timing on this question, Sound and Vision talked about this exact thing. I have also known others that have down this too.
You'll definitely want a tab-tensioned screen that's for sure. Non tab-tensioned screens always end up developing waves in them. I do know of one person that painted an inexpensive electric retractable screen and they say it reduced/eliminated any wave issues because the paint added weight to the screen and made it a bit thicker.
Long term though I am unsure how it will hold up. I know he's had a painted retractable screen for a year now, but over time paint does become brittle and my fear is after a couple of years it will start to crack and peel from the constant rolling and unrolling. As I mentioned, right now the reports of painting a retractable screen are positive, but it's even further down the road that concerns me.
A plain jane electric retractable screen is rather inexpensive now days, so I guess the mind set could be looked at that say three or five years down the road if it does end up developing problems you won't have a lot of money in it and can just replace the screen, but that can be a pain too.
A nice molding valance can hide the retractable screen and most people would not even know you have one until you drop it down over the plasma. Depending on how you have your speakers now, you may need to relocate them so the screen doesn't cover the speakers when it is down. That or you'll need an AT screen, and that will up the price a bit.
All in all it is definitely do able though.
What will really be interesting is S&V says a dual setup like this is "easy it is to assemble a terrific dual-display system for well under $10,000", and I say it can be done for under half that. They state that an entry level 720p projector is around $2grand, and that's about double what a good 720p projector can be purchased for. They also say flat panel sets bigger than 52" can exceed $8,000, which is true, but there are also sets $2,000 and under that perform very well... so their price list is a bit high in my opinion.
I have a dual display setup but I went with a fixed frame screen that sits above my 55" SXRD. I had to build a custom stand to lower the HDTV some so it didn't extend into the projector screen area. I also have the luxury of having 9' ceilings. Even though I opted for a fixed frame, my dual display setup comes in at around $3,300, so it can be done well under the $10K mark S&V talks about.