That information can be found in Internal Revenue Code Section 402(g). Also see IRS Publication 571.
If an individual participates in multiple retirement plans, the aggregate salary deferral among those plans for the individual cannot exceed $15,000 for 2006, plus catch-up contribution of $5,000.
Individuals are subject to one catch-up limit, regardless of the number of plans in which they participate.
For more insight, see Making Salary Deferral Contributions - Part 1 and Part 2.
This question was answered by Denise Appleby