Sonic may not team up with other playable characters in his upcoming downloadable adventure, but Sonic Team and Dimps are combining forces to develop Sonic the Hedgehog 4: Episode 1. Sega today confirmed to Joystiq that the two developers are working together on the title, plainly stating, "Sonic 4 Episode I is being developed by Sonic Team in cooperation with Dimps." The question as to who is actually making the title for Sega was raised when Xbox.com listed Dimps as sole developer on the game's XBLA info page.
This isn't the first time that Dimps has partnered with a well-known team to develop a game based on a classic franchise -- it worked with Capcom to create Street Fighter IV -- nor is it a stranger to the Sonic universe, having worked on the Sonic Advance series for GBA, Sonic Rush and its sequel for DS and level design for the Wii version of Sonic Unleashed.
[Thanks to Justin for the original tip!]