I think Sebastian's response is appropriate in that the funcitons mentioned are for ease of management of a DNN instance, not for complete cloning of sites. That said, I have given this a lot of thought and have a "working concept" in mind of how you could move a particular site with tons of pages, images and files to a new DNN instance.
Being a much less than average programmer (code hack really) I often find "easy" ways to use existing user friendly functions to accomplish my goals. In this case what would be wrong with simply zipping up the entire folder structure of the instance, backing up the DB into a zip as well, then installing it to another server.
Now that I have all of the instance at it's new home, I could simply delete all other sites, rename the DB, Login to the DB, give it a new password, change the web.config strings to match, and wallah!, the site is moved. Shouldn't take more than half an hour to an hour, but if the site is that big, and that important, should be worth it.
If I want to have a "clean" database to work with, I could simply create a new one and "import" the data from the old one, using SQL to reattach the Login User then do the above and again, I have the same end results with a new DB.
I may be wrong and haven't done a "real world" on this yet, but seems to me it should work.
Clay