For HDTV/Sports viewing with some ambient light, considering that the VW40 isn't that bright in brightest mode (if you want fairly good color), I'll stick with the same screen recommendations above.
I was able to do a lot of viewing in my testing room filling all of the 1.4 claimed gain, Carada Brilliant White surface. Every DVD I looked at looked great, no sense of being short of lumens at all.
In my theater (still off-white walls), using the Firehawk - the relatively brightest of all the HC gray screens, I was also very comfortable at 110" diagonal watching movies at night, with no ambient light issues. Football, on the other hand, during the day, with my shades leaking some ambient light, was just acceptable at that screen size. My JVC, with its extra 30%+ lumens was able to do equally well in its Dynamic mode filling the full 128" diagonal (as would be expected).
I must add that the Sony looked great on the Firehawk, with the projector showing rich colors and an exceptional feel of depth. And the Firehawk helps out significantly with side ambient lighting issue.
ony engineers and product managers must have liked the Firehawk too. Sony got together with Stewart last year to create a special version of the Firehawk, for the Sony VW50, and it's called the Firehawk SST. Now there isn't anything really special about the SST relating only to the Sony.
The SST has slightly lower gain, and works better than the Firehawk G3 (the one I use), if you are placing the projector fairly close to the screen. Stewart recommends the SST instead of the G3, if you are filling a 100" diagonal screen from 16 feet or less, as it will show less roll-off in the corners. That makes sense, so if you are mounting the Sony anywhere so that you have the zoom set for the middle of its range to full wide angle, the SST is the better choice.
Of course all major screen manufacturers have white surface screens, but if you do choose a high contrast gray, the Firehawk is the way to go, as it is the brightest (to my knowlegde) of the typical HC gray variety screen.