Maybe I'm a little old-school (I'm willing to accept that), but I just can't fathom putting important data on a flash drive. Maybe it's because I associate them with older removable media like floppy disks, zip disks, and CDs. All of which have burned me once or twice when you pick them up some time later looking for data. Plus since they are so small I don't think you can ignore the "lose-ability" of these things. I suppose you could mitigate that by having them in a "backup" drawer or cabinet. And the limited size of both flash drives and CD/DVD leads to having a bajillion disks lying around if you have a significantly sized set of data.
I somehow always forget about the free online storage like Skydrive. And there are other storage sites like Mozy too.
Those sites that don't directly support encryption you can always encrypt the data yourself before uploading (although that does add another layer to your process).
Like RebateMonger pointed out, you always need multiple copies. Just one copy is not a "copy". I've had friends take their photos from their primary hard drive and Move them to an external thinking it's backed up. It's not. In a best case scenario you have your original, a local copy, AND an off-site copy.
As far as will these companies go away, it's always impossible to say. I chose JungleDisk which uses Amazon's S3 service for actual storage, primarily because they used Amazon's service which is not likely to fail or go away anytime soon. But can anyone say that even Amazon will *never* go away? As big as they are, they might go bankrupt some day or decide it's not profitable and seel it off, or just plain stop offering the service. So the best thing to do is have multiple sets of your data.