Being promoted to MOSS Farm administrator in Central Administration Application still doesn't mean one may freely use also STSADM utility on this farm.
Promotion to farm administrator registers a user for use MOSS config database but not the Admin Content database. You need access to Admin Content database to be able to use STSADM - at least dbdatareader and dbdatawriter role.
Unfortunately, there are number of operations not available from Central Administration, but highly required to perform effective farm administration. One sample is adding a MOSS solution to the farm: you cannot do it from CA - only STSADM -o addsolution helps.
So, as far as you consider to use STSADM utility to manage your farm - be sure you have enough privileges for MOSS SQL database.