Microsoft have perhaps just set it to update periodically based on a timer. Vista used to lose the plot sometimes and not keep up to date with the desktop icons, especially if devices were being plugged in and out. In fact icon management on Vista was pretty crappy anyway - I've often changed the icon of a program and it refuses to show the updated icon even if the screen was refreshed - Windows seems to cache them and not update the cache when necessary. It has often taken a reboot for Vista to notice a changed icon.
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I've left WP indefinitely.