Re: failed to fetch cdrom, how bad is it?
How bad is it? Well, the "W" at the start of the error message stands for "Warning". In other words, it's not really an error message, just some information.
If you don't want to let Apt use your Karmic CD as a source for packages, just untick the CD on the first tab of System > Administration > Software Sources.
I try to treat the cause, not the symptom. I avoid the terminal in instructions, unless it's easier or necessary. My instructions will work within the Ubuntu system, instead of breaking or subverting it. Those are the three guarantees to the helpee.