Here is what I have found out about the game so far:
Release: Dec. 17, 2007 in North America
IGN Says

quote IGN

With MX Vs. ATV Untamed's December release looming in the distance, THQ has released a list of riders, music and sponsors moto fans can expect in the racing game's final build.

Twenty racers, more than 20 songs and close to 30 sponsors will be included in the title that features the legendary Rhythm Racing engine, which allows for new stunts and techniques while retaining the feel of off-road racing. The DS will also offer multiple classes of dirt bikes and ATVs along with stunt challenges, stadium racing and four-player wireless racing.


MX Riders

Billy Laninovich Josh Hill
Chad Reed Ricky Carmichael
Davi Millsaps Ryan Villopoto
Ivan Tedesco Nathan Ramsey
Josh Grant Grant Langston
ATV Riders

Keith Little Kory Ellis
Harold Goodman Mark Kendall
Dustin Wimmer Jason Luburgh
John Natalie Jr. Jason Dunkelberger
Pat Brown Dana Creech

Soundtrack

"On Edge" Authority Zero "Side of a Bullet" Nickelback
"12:34" Authority Zero "Dinosaurs Will Die" NOFX
"Social Suicide" Bad Religion "Soulcrusher" Operator
"Hearts and Stars" Cage 9 "Disconnect" Pennywise
"Shapeshifter" Celldweller "Loose" PRIMER 55
"Guarded" Disturbed "Face Down" The Red Jumpsuit Apparatus
"Kwad Up" Kasper "High of 75" Relient K
"General Admission" Mower "Collapse" Saosin
"Happy?" Mudvayne "I Don't Like You" Electric Six
"Secret Weapon" MXPX "The Ripper" The Used
"House Of Wolves" My Chemical Romance "Lobotomy For Dummies" Zebrahead
Licensed Sponsors

Acerbis Fox Racing
Alpinestars Hinson
AXO Kicker
Bell Lone Star Racing
Black Flys M2R
Factory Connection Maxxis
Fast50s Moto XXX
FLY No Fear
Oakley Shoei
Ogio Smith
O'neal Spy
Parts Unlimited Thor
Pro Circuit Troy Lee Designs
Scott Von Zipper
Shift MX