Assuming your document allows rollovers from all sources, this chart can be extremely helpful in sorting through the complicated variations of rollovers from plan to plan.
http://www.irs.gov/pub/irs-tege/rollover_chart.pdf
The Plan Administrator has discretion under Section 4.05 to limit the types of rollover contributions accepted by the Plan and must use that discretion in a consistent and non-discriminatory manner (PPA document).