Any Media is Susceptible to Damage
You raise a good question regarding the reliability of SDHC memory cards. I read somewhere once that you should treat the data on your computer as if it didn't exist. In other words, anything worth backing up should be backed up on a regular basis. It sounds like you've been quite diligent in this area and congratulations are in order!!!
The truth is that any type of media is susceptible to failure. Hard drives, DVDs, Tape, ZIP, Flash and yes, SDHC cards. The good news is that as you mentioned in your question, the cost of storage media of all kinds has come down to a level that we can and should backup our data on a regular basis.
The point I want to make here is this. If your data is worth backing up to begin with, you should back it up twice. I would never trust just one source to backup all of my data. So I would recommend that you backup your data once with the tools available through Windows Backup. This will allow you to create a Restore Disk and backup your entire system. In addition to this, I would keep a small partition on my hard drive to backup data only (pictures, music, documents, etc...). Finally, I would use an external backup for the same data on a separate hard drive, flash drive, DVD's, etc....
When choosing the media I'd use for an external backup, I personally don't think I'd use a card. I've seen too many cards go POOF!!! Once the card's been shorted, exposed to the elements, or many other potential threats out there, if the data is corrupted it's gone. You might consider Blu-Ray disks if you're using a desktop or laptop that can write to them. But I believe an external hard drive is the safest and easiest to manage among all of the possibilities.
SDHC cards are really handy for moving large amounts of data from one place to another (like from a camera to a computer hard drive) but as an archival tool I think I'd sway away from it.
Hope this helps