Brian, Magnets are being used extensively in mounting objects in the museum world. There are applications for their use in framing.
The primary advantage is that there is no altering of the art or object when using paired magnets to hold it in place. The one magnet needs to be fixed somehow to the mounting substrate, and the second traps the object to the first. Sometimes the second magnet shows, and sometimes it can be concealed.
These magnets are very small for the most part. I have a prescription pill bottle with about 600 neodymium magnets in it. Some of the 1/8" diameter magnets have in excess of 3lbs of pull.
Gwen Spicer wrote a book documenting use of magnets: "Magnetic Mounting Systems for Museums and Cultural Institutions".