How do I track the number of currently associated clients on cisco
series 350 & 1200 access points using MRTG (SNMP Graphing Tool)?

Two things you need to do:  Find the MIB for the currently associated
clients from your Cisco AP.  They should come in the form similiar to
"1.3.6.1.4.1.9.9.273.1.1.2.1.1"  MIB can usually be found by enabling
snmp on the AP and using snmpwalk tool:
  http://net-snmp.sourceforge.net/tutorial/commands/snmpwalk.html

Second is to insert the new MIB to your MRTG running configuration,
just make a copy of a existing mrtg.cfg file and change the Target
line similiar to:
  Target[10.0.0.1]: 1.3.6.1.4.1.9.9.273.1.1.2.1.1:community.string@10.0.0.1

See http://www.enterastream.com/whitepapers/mrtg/mrtg-manual.html#t9

That's the trouble, I can't seem to figure out which SNMP value is the
current number of associations... nor do I know what MIB that value is
contained in.  I'll continue to search though. Thanks for the tip,
I'll give it a shot!

jw

Try MIB 1.3.6.1.4.1.522.3.12.1.8.0, it should work for 350 & 1200.

see:
http://www.cisco.com/pcgi-bin/Software/WLAN/wlplanner.cgi

I will, thanks for the tip...