Since the 2009.02 release we have included a version/appliance specific package called turnkey-appliance-version in all appliances, for example: turnkey-core-2009.10

One of the main reasons for including this package is for versioning as you described in your post. It also includes the raw appliance changelog installed at /usr/share/docs/turnkey-appliance-version/changelog.

Displaying the version can be done as follows:

# dpkg-query -W turnkey-*
turnkey-core-2009.10

Hope the above helps.