Thanks to this blog post and metalink doc 741004.1, learned some fun stuff about how an Oracle agent determines the amount of free memory available on a Solaris host.

ORACLE_HOME=/u01/app/oracle/product/agent11g
export ORACLE_HOME
PATH=$ORACLE_HOME/bin:$PATH
export PATH

$ORACLE_HOME/bin/nmupm osLoad | awk -F"|" '{print $14}'

Now how to figure out why this would be so wildly different than vmstat...