To anyone else who uses a custom Google apps domain, it looks like Drive may not be available for a few more weeks: http://support.google.com/a/bin/answer.py?hl=en&answer=2...

Edit: Not so. You can enable the "rapid release" track for your domain by following the instructions here: http://whatsnew.googleapps.com/choose-release-track It may take "up to a day" for the setting to take effect, but once it does your domain's users should be able to opt into Drive right away from https://drive.google.com/start