TrueCrypt with Dropbox is probably the most secure way to store data encrypted. However, each time you change even one byte of what's in your encrypted container you'll have to re-upload the whole thing. That isn't very convenient unless you're storing a very small amount of highly encrypted data.
There are some secure alternatives to dropbox (see here: http://skeptu.com/secure-alternatives-to-dropbox) which use software to encrypt your files before they're sent to the servers, rendering them unreadable to the company you are storing them with.
The one advantage your proposed solution has over them is that they are mostly closed source, meaning that they theoretically COULD insert a back door in to their own code. That is less likely with truecrypt (it's open source + the code has been eyeballed a fair few times), although still conceivably possible. For the ULTRA paranoid I would advocate your solution.