Some of those 'fun surprises' if the disk was not detected were an error (code 42) when trying to save, a high level Rich Bandit attacking early in the game, and a couple thousand extra skill points given upon reaching level 15.
Did you try contacting
Securom? They have a utility to remove / update / reinstall their drivers, and if you have more than one optical drive I've seen the lowest drive letter one recommended for use, with comments that Securom can be picky about that.
Steam's DRM is certainly more appealing than some of the things done in recent games (limited installs, constant internet connection required for a single player game, etc), but digital distribution is not without potential problems.
I prefer having a physical disk and manual, but for new releases (after I get around to building a new computer) I intend to start paying attention to the DRM used. The only problem in the past I have had with DRM was a minor annoyance, which was later removed in a patch.
