This is an interesting idea for implying a hierarchy to a set of tags based on the order:
An alternative Approach to Tagging – ThinkPHP /dev/blog: “Instant Hierarchy
A method that combines the flexibility of tagging with the search-narrowing power of a deep hierarchy is to combine the tags to an ‘instant hierarchy’: The user chooses from a pool of keywords. Like in tag clouds, he gets to see some items then and a list of subsequient keywords. He can then choose a second keyword and get the items that are tagged with both chosen keywords, and so on. This instant tree is at it’s deepest in the path of the item with the most tags. “