I needed an Umbraco data type to store a GUID so that there could be a token to ensure a link created for a QR code was from a link we had made and not just guessed. The GUID goes into the query string and if it's present in the request then the application can do it's thing. If you ever need to cancel all the links in the wild for some good reason then you need to regenerate the GUID. The IA stays the same but the token is updated.
This wouldn't have been possible without this excellent Nibble.be article, that explains how this all works.
This uses .net 4.0 run time but makes no reference to the schema so should be fine in any version of Umbraco 4+