...it could be the colour wheel "running in" and becoming that tiny bit smoother / faster in its rotation.
Equally unlikely. The rotation of the colour wheel has to be very tightly controlled so as to synchronise exactly with the image applied to the DMD. If it didn't, then the colour balance of the image would go very wrong. And the speed is determined by the specified behaviour of the TV, wheel, DMD. The video processing will send sequential images to the DMD at a given and fixed rate*. And the wheel has to spin to coincide exactly with this fixed rate.
....I assume that if the dlp chips become cheap enough...then future rpro sets might incorporate 3 seperate DMDs
projectors have been available for a little while and, more interestingly, have just been re-priced to about half their original price. It isn't the DMDs that are costly, I gather. It's the other optics (prisms, colour filters, whatever) that are harder to produce than those used in 3LCD systems. That's their excuse, anyway.
* In a 6x system, for example, this rate, for PAL video, is 6 x red, 6 x blue, and 6 x green components (i.e. a total of 18 frame refreshes) in every 1/25 second (frame of video). In the same system, if the wheel has 6 segments (2 of each colour), then it must do exactly 3 complete rotations in every 1/25 second. Hence, the wheel presents the DMD with 3 x 2 = 6 occurrences of red, blue and green, per 1/25 second (video frame).