I had misgivings about Dropbox too, which is why I only sync one file, namely a 1GB TrueCrypt archive with all my data inside it. I hear Dropbox added TrueCrypt support in some fashion in their latest release, but I haven't investigated that yet.
Given foreseeable advances in technology, along with unforeseeable breakthroughs in both technology and cryptoanalysis no one can really say how long it will be until your data is compromised.
But what we can probably predict with some certainty is that at least some of these data storage companies will keep the encrypted data you send them for as long as they possibly can. And they may even share it with others.
http://www.arg0.net/encfsintro
Encfs encrypts file by file, so you do not have to worry about multiple people editing (and possibly corrupting) one giant file.