wait...
In my DMG, NONE of the armor items are half original weight, they are all less than half. Mithral shirt is listed in DMG at 10 lbs, standard is 25. Lg mithral shield is 5 lbs in the DMG, but 15 standard.
This still doesn't explain why the SRD and DMG make no mention of mithral altering the weight of armor and shield items.
Hey WotC, what gives??
--- John