Yes. To say the least, we are VERY cautious and paranoid ;) To date, we have never lost a single file. Typically (at most weddings) there are two photographers, so we instantly have a backup person in case of any issues. Then each photographer has two camera bodies, in case a camera malfunctions. Inside each camera body are two card slots. What this means is that we photograph on two cards simultaneously, so if one card fails, it is saving to another card at the same time.
After the wedding, the cards are downloaded and backed up on two separate hard drives. That way, the images are in two places.
The end results is that it would take two photographers, four cameras and eight cards all failing at the same time to lose images. With that said, there can be a blurred image here and there when capturing moving subjects. Or rare instances where the flash could not recycle quick enough to capture an image. But those would be a per-image scenario, rather than an entire event not being covered.
If we were to ever lose an entire event (which simply won't happen), we would of course refund every penny paid and would probably move to Canada ;)