I have my .bitcoin folder set to sync with Ubuntu One.
Is this a vialbe backup method?
Is this a safe thing to do?
First of all - create separate user for bitcoin with encrypted home folder. When you need to run client - switch to this separate user account, do transaction and then log out from this user account. This way you may be sure that no files left unencrypted (remember, if someone will get root or physical access all unencrypted files are accessible).
Second - in order to backup to ubuntu one - do so with first encrypting wallet.dat with your own pgp key. Make sure you can encrypt and decrypt it first of course. Use very strong and long password for your private key.
If you hold large amounts - consider creating "savings wallet". I.e. separate wallet.dat which is encrypted and contained in something. For example you can cerate virtual machine with ubuntu and put your "savings" wallet there. Then shutdown machine, encrypt it with pgp and put on ubuntu one.
ps: be paranoid.