I also did a truss to see what the threads were up to, and here's what I got:
bash-2.05$ sudo truss -p 26891 /1: ioctl(105, DP_POLL, 0xFFBFE8B8) (sleeping...) /2: lwp_park(0x00000000, 1) (sleeping...) /3: lwp_park(0x00000000, 0) (sleeping...) /4: lwp_park(0x00000000, 1) (sleeping...) /5: lwp_park(0x00000000, 0) (sleeping...) /6: lwp_park(0x00000000, 1) (sleeping...)
On closer examination all of the above look normal; but note that hanged backends usually unstick if a signal is received.
Bill