Installation, i will only provide bash commands, don't have time to write guide ATM.
Uses check_apt from http://nagiosplug.sourceforge.net/

I have included a precompiled file so people without compilers on their servers can use it.
It's compiled for i686 with the following flags:
	CHOST="i686-pc-linux-gnu" 
	CFLAGS="-march=i686 -Os"
	CXXFLAGS="${CFLAGS}"

On the host to be checked:
-----------------------------------------
mkdir /usr/share/snmp/custom
mv check_apt /usr/share/snmp/custom

edit file: /etc/snmp/snmpd.conf (or wherever it is on your system, i use Debian Etch)
add a new section:
	# Custom commands
	exec aptgetsnmp /usr/share/snmp/custom/check_apt
save and exit

restart snmpd server:
/etc/init.d/snmpd restart

Now run the script remote-check_apt_snmp on the nagios server:
Example:
[root@nagios] test # ./remote-check_apt.php -H x.x.x.x -C public
APT CRITICAL: 1 packages available for upgrade (1 critical updates).

Debugging, having problems? check the output of:
	snmpwalk -v 1 -c public x.x.x.x .1.3.6.1.4.1.2021.8
where x.x.x.x is your host to check. and look for the apget string