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Problem:
If the loop() function of asyncore is invoked with poll
rather than select, the function readwrite() is used
when I/O is available on a socket. However, this
function does not check for hangup - provided by POLLHUP.
If a socket is attempting to write, then POLLOUT never
gets set, so the socket hangs. 
Because poll() is returning immediately, but the return
value is never used, asyncore busy-loops, consuming all
available CPU.
Possible solutions:
The easy solution is to check for POLLHUP in the
readwrite() function:
 if flags & (select.POLLOUT | select.POLLHUP):
 obj.handle_write_event()
This makes the poll work exactly like the select - the
application raises a socket.error set to EPIPE.
An alternate solution - possibly more graceful - is to
invoke the handle_close() method of the object:
 if flags & select.POLLHUP:
 obj.handle_close()
 else:
 if flags & select.POLLIN:
 obj.handle_read_event()
 if flags & select.pollout:
 obj.handle_write_event()
This is incompatible with the select model, but it
means that the read and write logic is now the same for
socket hangups - handle_close() is invoked.

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Perhaps, it would be better to raise exception:
def readwrite(obj, flags):
 try:
 if flags & (select.POLLIN | select.POLLPRI):
 obj.handle_read_event()
 if flags & select.POLLOUT:
 obj.handle_write_event()
 if flags & (select.POLLERR | select.POLLHUP |
select.POLLNVAL):
 obj.handle_expt_event()
 except ExitNow:
 raise
 except:
 obj.handle_error()
...
 def handle_expt_event(self):
 err = self.socket.getsockopt(socket.SOL_SOCKET,
socket.SO_ERROR)
 assert(err != 0)
 raise socket.error, (err, errorcode[err])
Since asyncore closes socket in handle_error, this solves
the problem too.

The solution suggested by klimkin seems to have made it into revision 35513 as a fix to bug #887279.
I'm not sure that this is necessarily the right solution to this bug or #887279, as a socket disconnect isn't necessarily an error condition, otherwise .handle_close_event() shouldn't exist for select-based loops, and it should always be an error.
Suggest switching to the last if clause of readwrite() to...
if flags & (select.POLLERR | select.POLLNVAL):
 obj.handle_expt_event()
if flags & select.POLLHUP:
 obj.handle_close_event()

Fixed for 2.6 in changelist 64768.
Fixed for 3.0 in changelist 64770.