I love that they openly in this post discuss that the end-user can use TrueCrypt if they desire:
"For users who feel more comfortable managing their own encryption keys, we recommend using products like TrueCrypt to store encrypted volumes within their Dropboxes. Those users will unfortunately lose access to some functionality, but we leave this decision to the user."
It's the sign of a good company in my opinion that they don't treat their users as unable to understand such things.
Except that when you use TrueCrypt most of the functionality of DropBox stops working; you can't access your files through the web interface anymore (duh), but most importantly, you can't use the volume on two machines at the same time.
I've used TrueCrypt volumes with Dropbox before, and when I encountered a situation where I wanted to access the volume from two places, I took the sensitive data from Dropbox and just exposed it on my private network where I have more control over access. As far as I can tell, there isn't a great way to keep high security and flexibility when using sensitive data with Dropbox, but keeping sensitive data on the cloud is always a risky affair anyway.
"For users who feel more comfortable managing their own encryption keys, we recommend using products like TrueCrypt to store encrypted volumes within their Dropboxes. Those users will unfortunately lose access to some functionality, but we leave this decision to the user."
It's the sign of a good company in my opinion that they don't treat their users as unable to understand such things.