You can not catch a Mew natively in the two new Pokemon games; Diamond and Pearl. However, all is not lost!
Several months ago Toys-R-Us had a Mew give away deal, I was there. I acquired many Mews on that day and now you can benefit from my gain.
Right now I have one level 10 Mew in my Pokemon Pearl game and I am willing to trade it away for a Dialga with the Admantium Ball/Orb thing that he uses. This Mew has never been trained or anything, he is in the same state he was when I got him at the Pokemon event.
ID No. 06930
HP - 41
Attack - 23
Defense - 26
Sp. Atk - 25
Sp. Def - 26
Speed - 27
Attacks: Pound and Transform
This Mew has been traded
If you want him, and you have a legit copy of Dialga with his orb then he can be yours.
Leave a note in the comments section and we can work something out.
PS: My friend code is: 0559 3242 3898
UPDATECheck out this thread over at Serebii Forums. Somebody got a hold of one of the Toys-R-Us cartridges and is giving away a ton of Mews!