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I can confirm the primary symptom on WinXP.  For the 10 
seconds http_test.py runs, CPU is nailed to the wall under 
current CVS Python, but is a trickle under 2.3.4.

asyncore appears to be irrelevant.  "The problem" is that the 
socket associated with __main__.http_request_handler 
always comes back in the w set from select.select() under 
current Python, but the select times out under 2.3.4.

More detail:  under current Python, dumping some prints in 
asyncore.poll() delivers a huge number of repetitions of this 
info:

socket map is
{1940: <__main__.http_request_handler connected 
127.0.0.1:3455 at 0x9da800>,
 1948: <__main__.AsyncHTTPServer listening :40004 at 
0x9d7fa8>}
r, w, e passed in is [1940, 1948] [1940, 1948] [1940, 1948]
r, w, e returned is [], [1940], []

Under 2.3.4, there are only a few lines of output total -- 
select() doesn't believe the sockets passed to it are ready to 
read or ready to write.

So the difference is in socket behavior, or in how the layers 
around asyncore here are using sockets.