I don't understand why they would want to go grey when they are zooming in. They have the tile for the old zoomlevel, they should just scale it until new data comes in (and overlay the new data on the old). This is exactly what happens in the browser when you zoom (as long as you're not zooming, say, 6-7 levels).
Also, I think it would be better to render the whole page at once, with a width of something nominal like 800px or something, and then you can pan and zoom (with or without smart zoom) super fast in hardware. It has always been a dissapointment that the zooming is not as smooth on Android because the browser tries to re-flow all of the "fluid" pages to it's new width and height. (it also breaks some pages, where they try to float something to the bottom of the view, but the view on Android is just slightly larger than the screen, so it's unreachable)