Lightroom being (at it's heart) basically a database, the whole point would be to have ALL your images on the one catalogue (=database, or library in version 1). I have my photographs spread over more than one drive but all in one catalogue. If a particular drive is not plugged in, the images show in the catalogue but I cannot edit or print them. I can even have images on a CD in the catalogue, no worries, even if the CD is locked away in its case.
The way to make this easy, is to ensure there is a main folder within the catalogue for each drive, with the folders on that drive listed as children of the drive's main folder. It is very, very easy and effective. In practice, I don't bother putting CD/DVD backups on the catalogue but they are named to match backup folders so would be easy to reimport from.
It's one of the reason's I love Lightroom. My photos can be anywhere but I can access them through the one programme. By the way, this should also work for a large business with a network, with each person having his or her own "master" folders, though I haven't tried it.
Incidentally, there is a big improvement here with version 1.1. With version 1, if I opened Lightroom without one of the drives plugged in which had been plugged in the previous time, it took a while for the programme to sort out the file locations. Now it is instant, thankfully.
Finally, I regularly backup the main catalogue and previews to an external drive. Just in case. The catalogue, by now, represents quite a lot of work!
Hope this helps,