The reign of the Glitches is back!!
Here's how our good ol' Missingo made its re-appearance in the Diamond & Pearl games just recently launched. According to the GameFAQs thread, the glitch is caused by having only 1 non-KO'd Pokémon in your party, and an EGG in the second slot, then entering a 2v2 battle. If you have D/P, you could probably try it yourself.