Was in the nether, pushed a donkey through a portal to the overworld.  Followed in.  Donkey was suffocating in stone adjacent to the portal, and it died.

No item drops could be found on either side of the portal.

Additional detail: portal was built on obsidian above lava at Y=10.  Maybe the items were sent down into the lava?  If so, that doesn't seem the right direction for items to travel.