This is an idea that has been around in various forms for a number of years. A number of other sites have popped up but like a number of people have already said, I don't trust random third parties with the keys to my online life.
Two keys, you put one on memory sticks which you give to friends/family you trust. In the event anything happens to you the other key is sent to those people allowing them to decrypt it. Service can't access your data as it only has one and same for trusted person. I'm sure something like this already exists (and tbh the level of effort required to set it up pretty much makes it unlikely to catch on) but it is theore…
http://www.freebsd.org/cgi/man.cgi?gshsec
Want to set up a "2 of 3" (or similar) scheme? You could use, for example, a three-disk RAID5 using USB flash drives.