I finally took the time to search through the code and find why this message was shown every time I login. It turns out the database table 'og_config_options' contained a record with name=upgrade_last_check_new_version and value=1. I changed the value to 0, and I don't get the message anymore.
Now I see why this was happening too...
I was using 1.7.3.1 on my old server (upgrade to 1.7.5 was available). I backed up my database, manually installed 1.7.5 on my new server (and copied appropriate data files from old server), restored the database to my new server and upgraded the database.
Apparently, the only way the 'upgrade_last_check_new_version' record is changed back to 0 (no upgrade available), is after an upgrade, since I couldn't perform an automated upgrade through the application, this value was not being updated, but every time I logged in, it was being checked and displaying the message to me.