zfskern{txg_thread_enter} thread using 100% or more CPU

Hi,

Recently on multiple systems running CURRENT, I've been seeing the 
system become unresponsive. Leaving top(1) running has lead me to notice 
that when this happens, the system is still responding to ping and top 
over ssh is still working, but no new processes can start and switching 
to other tasks doesn't work. In top, I do see pid 17, 
[zfskern{txg_thread_enter}] monopolizing both CPU usage and disk IO. Any 
ideas how to troubleshoot this? It doesn't appear to be a hardware issue.

Steve