all widgets are pinned to a grid and with a couple of x and y formulas you can let them slide. Giving the widgets a higher x and y formula, make the backgrond seems to come loose from the widgets, making them laying on top of them.

If it were really a bitmap made by the n900 then it would for sure be very stuttering and it wouldn't be so smooth. making a screenshot hitting ctrl+p results in a 5 second full load of the CPU. imagine that you scroll 10 times through all homescreens in a few seconds. the n900 couldnt keep up with that for sure.

As i said earlier the widgets move apart from the background, so again its not possible with a bitmap