Great, now only if they actually encrypted my files.
With a TrueCrypt volume or other encrypted file solution on top of Dropbox, you have to resync the entire multi-GB volume any time a single file in there changes, since to Dropbox it's just one big file. (Another option is to use something like an OS X sparsebundle -- encrypted data banded across many files -- but God help you if you have two computers reading/writing from that sparsebundle at once.)
I've started using SpiderOak and it is quite efficient even though the data is encrypted such that the server admins couldn't see your data even if they wanted too. https://spideroak.com/engineering_matters SpiderOak also offers two-factor auth. (The SpiderOak UI, however, is fairly atrocious.)
But they do: store files and name them sequentially ;)
I think Encfs is the best solution, since it encrypts each file separately. Just mount the encrypted end over the Dropbox directory and the plaintext end somewhere else, and use it transparently.
However, like the other commenter, I still recommend encfs for most uses.
A service for syncing and sharing with built-in encryption might therefore me more convenient for most users. Spideroak and Wuala are two examples.
Worth doing for some files, yes, but still a pain. Per-file encryption would be ideal, but a monstrous pain to implement with TrueCrypt.
Dropbox is probably not crazy about widespread encryption because it would eliminate their ability to perform deduplication. Perhaps they could get around this if you had a special encrypted quota. For example, you have 5GB of space and 100MB of encrypted space.
I just modified a file in a 64MB truecrypt container and unmounted it. Dropbox took roughly five seconds to sync it.
The only real problem with truecrypt in dropbox is that you can't keep it mounted on multiple devices.