Only part of a truecrypt volume changes when you make a change to the data within. Since Dropbox intelligently syncs files in parts, only part of a truecrypt volume has to be re-uploaded when something changes -- not the entire multi-GB file. I know because I do exactly this with a 1GB file. It takes about 2 minutes to sync when I unmount the file. Dropbox is not re-uploading all 1GB.
However, like the other commenter, I still recommend encfs for most uses.
However, like the other commenter, I still recommend encfs for most uses.