The last Horcrux theory!
It's possible that it is The Goblet of Fire trophy cup. Remember in the 4th book when Voldemort kills Frank and he uses his death to create a Horcrux out of Nagini. What if he created a Horcrux out of Cedric's death?
It was said in 'Half Blood Prince' that Voldemort uses valuble possession as his Horcruxes e.g. Slytherins locket,Huffelpuff cup etc. The Goblet was the item that would bring Voldemort Harry and his body back. It is an important object and valuble to Voldemort.
Wormtails Silver Arm:
Now, remember Voldemort took the time and a lot of energy and power was shown making Wormtail's arm.
Voldemort made it after he came to power again, and since this is such an important date, a sort of important ceremony of his return, this arm represents this, as to mark the person who helped him come back.
If it was any other time, and Wormtail had broken or lost an arm, or something, Voldemort would not have been so 'nice'.
Remember, it is not all of Wormtail as it is a fake arm, made of silver. It shows such great creativity and cunning in the making.
We have not seen much of Wormtail either, since Voldemort's return. In HBP we learn he is 'stowed' away at Snape's house. He was complaining that he wasn't doing much, and that he was certainly not there to be Snape's slave.
Why would Voldemort care abot Wormtail so much that he is hiding, [i]safely[/b] tucked away in the secret area of Snape's house?? He doesn't - and he does. Not about Wormtail, but about the arm. The Horcrux.
The last Horcrux had to have been made after Voldemort came back to power, as it was such an important event for Voldemort. And the only thing, object, we see of when Voldemort returns, is the Silver Arm he makes for Wormtail.