In Windows 7, with Truecrypt 6.3a, and Window's user account control enabled (default) the mounted true crypt volume's security permissions default to read-only for the local computer's "users" group.
This assumes that you are NOT mounting the volume "read-only" in truecrypt.
Applications that are not "run as administrator" therefore only have user-level access and require UAC to prompt and you to respond in order to achieve administrator permissions, even if you are a member of the local administrator group. This works when creating a folder or renaming a file on the mounted volume, but when you open a file in an app, e.g., Excel, it sees the file as read-only and never prompts you to elevate permissions.
Temporary work-around is to edit the volumes WINDOWS file permissions after mounting and grant yourself full control OR full control to the USERS group.
Not sure if this can be changed in Truecrypt's mount procedure to automatically grant the logged-in user full control or if there is a better work-around.