I'm using JOSM 5267 on OS X 10.6.7 on Java 1.6.0_26. I've got it set with the OS X Look and Feel in preferences, and I get this slightly weird "flying text" thing. Various street names hover above the map and as I move around the map, the text moves away from the actual street. I've illustrated it with a screencap that I've put up on YouTube: pop it into 720p quality and full-screen it to see the issue.
I don't know if this is a bug or a known issue or not, but it's quite annoying. I don't seem to have it on my laptop running JOSM 5047 on OS X 10.7.4. I'll update that in just a second and see if it's still a problem.
Haven't seen this bug so far, can you try 5047 on your OS X 10.6.7 machine?
It's not a bug I've heard of before. If it's still an issue with the latest version of JOSM, you could file a bug report at the JOSM bugtracker for this:
You could look here whether any of the existing reports matches your issue first: