Back in 2006, Kuju (now Headstrong Games) the development studio behind the Wii's House of the Dead: Overkill and Battalion Wars 2, liked Sega's Jet Set Radio series. They liked it so much back in 2006 that they tried to convince Sega to let them create a new entry in the graffiti-thon for Nintendo's then-upcoming little white box.
Sega's answer was, obviously, no dice. Simply not interested.
Unseen 64 has dug up some concept art that Headstrong created for their denied pitch, and you can check it out below.
It'd be nice if this project was brought back to life, as Headstrong has shown with Overkill that they can take a classic Sega franchise and create a worthy new entry. Jet Set is also known as one of the first console games to really put cel shading to use, which would be fitting for Wii since it wouldn't require the raw horsepower of a photorealistic title.
Sega did reapply for the trademark back in 2008, but that doesn't necessarily mean that a sequel is in development, just that they want to retain the trademark. Then again, it doesn't mean there isn't.