If you do this, make sure you disable the TrueCrypt option which preserves the original modification time of the encrypted volume when you modify the container, otherwise Dropbox may not notice when you make changes.
The "preserve modification timestamp" option is part of TC's plausible deniability feature.
In theory, if multiple snapshots of truecrypt are maintained, I believe it must be secure, but did the designers of truecrypt keep that in mind? Does dropbox maintain periodic snapshots of uploaded files?
Unlimited versions within last 30 days for all free and paid users (including changesets where files are deleted), paid users can also pay extra to keep every version since file creation.
So if you were suspected of getting a secret file and placing it in a dropbox hosted truecrypt volume, there would be a record of the time/date of altering the truecrypt volume and the possibility to compare the differences between two versions.
If you're uncomfortable with dropbox, put a truecrypt partition right inside your dropbox folder.