When my party was disguised as Drow and then changed back by the Silver Dragon, some of their equipment disappeared...in particular, Nalia lost her robe of the good arch-magi. Is this a bug, or was it dropped somewhere in the Dragons lair and I need to troop back there to check?

That is strange but I've seen it happen if you have a full inventory and they try to give you stuff that your stuff gets dropped. I'd reload and decide how much don'tneed, don't want, or can stuff in your bag first to clear up some room. You really don't want to troop back into the Underdark after your disguise is lifted.

Certain summoned monsters also have the ability to destroy magical equipment. Be sure to kill off those guys as quickly as possible.

You are correct...on further reflection, we were attacked by a Drow party in the Underdark that summoned Nisshuri to attack my magic users. They hit Nalia a couple of times, and evidently destroyed the Robe of the Good Arch-Magi...Is this a bug? I thought they were only supposed to drain wands and the like...This sucks...I don't have a saved game that far back to reload. I take it there is only one Robe of the Good Arch Magi in the game? (the armorer in Trademeet)