I once head the forward tilt explained as part of the alternate extension mechanism. The trunnions in the wing, on which the gear swings, are toed out slightly ( toward the wingtip ) at the front. Thus, when the gear legs free-fall during alternate extension, aerodynamic forces help push them aft, helping the side and drag braces on each leg to be locked. If the bogey was level, or tilted rear wheels down, then the fuselage would have to have been bulged outward to accomodate the gear in it's retracted state. Not ideal drag-wise at all!
In their retracted state, they would lie further forward, thus countering any reduction in cruise trim drag achieved by pushing the c.g. aft.
That said, the 767 gear is a wonderful piece of engineering, in spite of the curious little flap on the after part of the wing where the cam on the retaction jack breaks the surface during retraction!
The forward tilt even allows a range of feeble excuses from the flight deck on the odd occasion the touchdown isn't as sweet as one would normally like!