I don't know anything specifically about ODBC, but in general, if there's a difference in the way your code works when you're logged in and when you're not logged in, the first place to look is in the environment variables. There could be an environment variable defined in your .bashrc that doesn't get set when the program's running as a cron job (for example).

Apologies if you're not running *nix and this makes *no sense* =)

HTH!