CDs and DVDs are simply inappropriate for archiving. They can't be trusted, even the best of them.
Outboard Firewire hard disks are now well under $1 / GB, almost as cheap as quality DVDs. 250GB drives can be had for under $200 - about 75 cents a Gigabyte.
I'd just like to point out that hard disks are at least
as unreliable as CDs and DVDs, and allows you to lose that much more data in one go.
They're not very tolerant of temperatures above 30 degreees C. Temperature changes are even worse, not just because there's metal inside, but also the condensation risk. It's fairly common for disks to fail if the temperature fluctuates too much, and if the drive operates for too long outside its operating parameters, it can perform erratically, leaving files in a corrupted state.
CDs and DVDs handle this better (but see my remarks below), and are also easier to store in places where you can avoid this kind of stress.
My strategy is to have all my files online and available, and a second Firewire drive off line (powered down) with the online content backed up to them. I make DVDs of my most important files and bring them to another location as "disaster" insurance, but not as my only copy.
I also make DVDs in addition to keeping all my images online, but I make two verified copies, before archiving them in fire-resistant safes in different physical locations.
The verification procedure in the DVD burner helps me identify glitches in the burning process.
I still haven't come around to introducing my scheme of adding checksums of every file to the DVDs and to disk, though. This would help me discover possible corruption quickly.
One thing that worries me about CDs and DVDs, is the accountability of CD and DVD production processes. Which kind of dye are they using? Is there aluminum, silver, or gold in there somewhere? What kind of plastic are they using? How about metal, dye and plastic impurities? Is their clean room production facility really clean? These factors affect reliability and durability to a significant degree.
As for hard disks, there are companies such as IBAS
(I'm not associated with this company) who can help with rescuing data even in cases where it's seemingly impossible. It's not anywhere near cheap, though.
On the plus side, working digitally does allow us to take all kinds of precautions with our data. There isn't only one set of negatives.