fork: Resource temporarily unavailable / cant start new threads

[mark]

I upgraded to 2.6.25.3-18.fc9.x86_64 fedora core 9, now I get this
error when I try to login to the box, kill a pr start a python app, or
do anything on a regular basis.

fork: Resource temporarily unavailable

I have over 10GB RAM free, and zero swap spaced used. The box is a
dual quad core Intel Xeon 5405 with 16GB RAM.

There is no error message in /var/log/messages or dmesg ...
how do I identify the problem?
thanks!

uname -a
Linux XXX 2.6.25.3-18.fc9.x86_64 #1 SMP Tue May 13 04:54:47 EDT 2008
x86_64 x86_64 x86_64 GNU/Linux


free -m
            total       used       free     shared    buffers     cached
Mem:         16086       3189      12896          0         42        666
-/+ buffers/cache:       2481      13605
Swap:         1983          0       1983


have only 505 processes running
ps aux | wc -l
505


uptime
 11:24:15 up 39 min,  1 user,  load average: 3.54, 3.47, 2.87

ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 137216
max locked memory       (kbytes, -l) 32
max memory size         (kbytes, -m) unlimited
open files                      (-n) 32768
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 10240
cpu time               (seconds, -t) unlimited
max user processes              (-u) 1024
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited
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