Interesting…
There seems to be no way to format an unformatted floppy disc under either Tiger or Leopard. But, even if a disc is pre-formatted in the "PC" format, you can use Disk Utility to "Erase" it to a Mac format (or from Mac format to PC "Fat" format).
I guess that's why they don't sell unformatted floppies anymore. (I did find that I still have a whole bunch of them though, so I going to have to fire up OS9 on one of my older Macs to get them formatted just so I can try to give them away.) 🙂
I did find that once a disc is formatted it does work properly with Leopard.
A quick test with TextEdit to save a document:
1) Choose Save… from the File menu.
2) Type a name for your file.
3) In the "Where" box, choose the name of the floppy disc from the drop-down menu (it will have a white icon next to the disc name).
4) Change the format of the file if you want something other than the default.
5) Click the Save button.
Opening a file works similarly, i.e. Choose the floppy in the Devices section of the sidebar of the Open… Then choose the file within the floppy.