From the rrddef.cfg comments:
*# Each section key is the column- or data-name reported from your
# client test.* Typically this is "cpu", "memory", "tcp", or some
# custom test you have created.
In your original script:
[ buda ext]# cat httpd_threads.sh
#!/bin/bash
THREADS_HTTP=`ps -ef | grep -c [h]ttpd`
*LINE="data $MACHINE.trends*
[httpd_threads.rrd]
DS:threads:GAUGE:5:U:U $THREADS_HTTP"
$BB $BBDISP "$LINE"
it appears that your data name is "trends". This is what xymon is trying to match in rrddef.cvf. The key you should be using in rrddef.cfg.
[trends]
-s 5
RRA:AVERAGE:0.5:60:576
RRA:AVERAGE:0.5:360:576
RRA:AVERAGE:0.5:1440:576
RRA:AVERAGE:0.5:17280:576
With that said, "trends" is already a column heading... I'm not sure that using it for your rrd key won't cause other problems in xymon. Maybe change it to "httpd_threads"?
If that doesn't work, try putting the -s in the default section "[]", and see if that works. You shouldn't leave it there as it will set all future graphs to a 5 second step, but it works there, then you know your problem is with your data name and key.