From what I have read, the job is Chris Brown's to lose. The fact that they signed Smith, however, does not indicate they are convinced he can carry the full load.
I saw a lot of Brown in college (Colorado alum), and he is a decent straight ahead runner. His great college production, however, was more a product of a very good offensive line then any extrodinary talent on Brown's part.
While I don't see him making it through training camp with a "feature back" role, he probably will take a majority of the carries this year simply due to lack of competition.
Keeper League (Keep 3)
QB - Daunte Culpeppe (Jake Plummer)
RB - Ahman Green (Travis Henry, Chris Brown, Onterrio Smith, Steven Jackson)
RB/WR - Marshall Faulk
WR - Derrick Mason, Chris Chambers (Plaxico Burress, Ashley Lelie, Marcus Robinson)
TE - Todd Heap
K - Jason Elam
DEF - Miami Dolphins