I think it's pretty clear in the modern NFL that only a very few running backs are effective past age 30. Thomas Jones is one of the only recent ones that come to mind, but he is a physical freak. Given the escalating salaries of QBs and WRs, expect continued reliance on platoons of younger backs (cannon fodder). Effective backs can be had in the 2nd and 3rd round and can be paid (relatively) cheaply. I'm not sure what the Jets are thinking with Tomlinson.
On a side note, rookie running backs shouldn't sign for more than four years on their rookie deals given their short shelf life.