Not sure if it's up to date, but the FAQ attempts to answer this question:
On Tue, Aug 31, 2010 at 1:04 PM, Allen Wittenauer
> On Aug 31, 2010, at 12:58 PM, jiang licht wrote:
> > Is the number of nodes to be decommissioned bounded by replication
> No, it is bounded mainly by network bandwidth and NN/DN RPC call rate.
> I've seen decommissions of like 400 nodes at once.