This is one of those questions whose answer depends on the DM.

It's my impression that most DM's divide XP from defeating monsters evenly among the group, except in very unusual cases. However, whether treasure XP is divided evenly seems to depend more on the DM. I personally award treasure XP depending on which characters actually get the gold and magic items. So if the players decide to divide the gold and magic items unevenly (or if thieves in the party steal an extra share), they get uneven treasure XP awards. In my case, the answer to your question would be, "Whoever keeps the armor gets all the XP for it and the rest of the party gets none (for that particular item)". Other DM's, I believe, would answer differently because they divide treasure XP evenly the same way they divide combat XP.

If you want an "official" answer for AS&SH, maybe Jeff will chime in. Basically, though, all XP awards are at the discretion of the DM... so I guess it's however you want to handle it!

From VOL. III, p. 266: EXPERIENCE POINTS (XP), gaining experience points:

XP Distribution: Experience point distribution can be a tricky subject, though only as complicated as the referee chooses.
Even Distribution: In the broadest sense, XP should be divided equally amongst the PCs, even when one character outshines the rest of the party.
Solo Distribution: Sometimes a player character will perform individual activity deserving of solo XP. Example: A thief sneaks ahead to scout the opposition and runs into a pair of orc guards. He fights them by himself and wins by dint of his blade before his allies arrive. In such an instance, the thief may be awarded solo XP.

My feeling is that the 1,000 XP for Dagon’s harness should be divided evenly amongst the party, unless one PC recovered it by himself.

I usually use even distribution because usually everyone's cooperated to secure the treasure, victory, etc.

I distribute experience points for treasure equally among the characters who secured it. The players are free to split the treasure among themselves however they like

I equate the experience with the fame/glory of retrieving the treasure. So, it’s split amongst party members just like treasure, unless the character won it single handedly... stealing the glory so to speak. Since the magic item itself is a reward, I don’t feel it necessary to otherwise reward a character for its possession.

We do it all pretty evenly with a couple exceptions.  Players who are exceptionally clever and/or do something really selfless or heroic or role play really well might get a bonus from the DM.  And you get xp for delivering or receiving criticals.  It's 50 xp per degree of crit delivered (2-5xdamage) and 100 xp per severity level of critical received.  So you could theoretically get 1000 xp for surviving a crit that inflicted 5x normal damage.