In all the following tests the red box has position:relative. To check its rendering it is sandwiched between two boxes (grey, zero margins, static position.)
With a slightly bigger than normal line height. In IE7-/Win the relative box is not fully rendered in the top part: there is a white stripe between the first two boxes.
with border
With a bigger line height. The problem is more evident.
with border
with margins
With hasLayout on the relative box, the problem disappears.