When I need to add a ribbon as a border to a cake that has been covered in fondant, can I use a straight pin into the cake to secure the ends?  If no, what can I use to secure the ribbon onto the fondant?  I am thinking edible glue might seep through the ribbon.

APlease don't stick sharp needle like things into a cake. People aren't real bright and might not realize they are there. You can certainly use a little dab of edible glue, or buttercream, or melted chocolate, something. And even if it does bleed a tiny bit, it's a tiny bit, who will really notice?

AI use double-sided tape and stick the ribbon to the ribbon :-) technically it could move around but i do it tightly and so far, it only moves when I want it to :-)

There's a back to every cake, thank goodness.  "No" to using pins.

..........When I need to add a ribbon as a border to a cake that has been covered in fondant, can I use a straight pin into the cake to secure the ends? ..........

never Never, NEVER!!!  What AZCouture said.

I use double-sided tape, or I take a needle and thread and run a straight stitch through the ends of the ribbon then pull it tight so that it's sewn together. That works really well becasue you know the tape won't let loose, but there are no pins etc to poke someone.