Yuu Mizushima has updated his blog with a picture from the cover of an upcoming script. The episodes title is simply "Riley and Lucario!" (ゲンとルカリオ!) as it's printed on the script book itself.

The episode will be the 108th episode of DP and it will most likely air in early January of 2009. Yuu Mizushima will voice Riley. His last role in Pokémon was Doctor Moroboshi from the Izabe Island arc in Advanced Generation.

"Riley is a Pokémon trainer from Sinnoh and he is one of five trainers who can join with the main character during an event in Pokémon Diamond and Pearl. Later, he can join the player in competition at Sinnoh's Battle Tower. He first appears at Iron Island. He uses a Lucario and shares similarities with the character Sir Aaron. Riley will give the player a Riolu egg after finishing the event." Bulbapedia

Below is a translation of Yuu Mizushima's blog post. Thanks to Sushi for the translation.
"I just came back from recording for a Pokemon guest character. You won't believe it, it's a human character! Not to mention, a handsome one! Lately I've been voicing so many non-human characters that I felt a bit nervous... I'll appear in episode 108 as Gen. Please look forward to it!"

Source: http://yumizushima.blog122.fc2.com/