Yes there is if you do /say @e @e when there is a bobber near you it says PLAYERNAME PLAYERNAME and unknown.
When I renamed the bobber using the command /entitydata @e[name=unknown] {CustomName:"Joe"} it actually renamed the bobber and the name was displayed above it.
And by the way in the wiki http://minecraft.gamepedia.com/Data_values/Entity_IDs it doesn't say anything about the fishing rod's bobber.
Please read this and try it on your minecraft before responding, thank you.
-Teo
Ok thx
Did you actually tried in your minecraft?
Using the following command: /entitydata @e[name=unknown] {CustomName:"RandomName"}
while there is a bobber near you?
This time seriously try it on your MC and then come back and tell me what you have discovered.
You will found out that a mystical name tag will magically appear from a mysterious world above the bobber.
Sorry if I sounded rude didn't mean to but I hate when people don't know what they're talking about.
If you want a video proof look at this http://m.youtube.com/watch?v=9StUwrTpS4Q a video by Dragnoz.