If done right, this could be cool. But I don't know that it'll be done right. With software, there are sometimes compromises made for political reasons or expediency. If Apple thinks there'd be pressure from enough customers, they might feel it necessary to put in a back door to any encrypted cloud volume so their people could help recover data for those customers that forget their password.
What I'd like to see in Lion is a move towards more security. Make it the default for people to not run their day-to-day stuff using admin accounts, for example - that'd do a lot right there. An option for full disk encryption, as someone has already mentioned... that'd also be great - assuming, again, there are no back doors or failsafe secondary methods of access. Making your keychain password different than your login password by default, instead of requiring the user to figure out how to do it. Stuff like that.