PDA Musician: PocketSynth Pro naming

Look at these two products: Pocketsynth and PocketSynth Pro. I wrote the original Pocketsynth in 1996, and the folks at PDA Musician wrote another product with the same name many years later, a product for Windows CE. Despite the fact that the program they wrote is probably very useful, it's unfortunate that they chose the same name. I suppose that I would take action if I were still supporting Pocketsynth, but since I'm not, I guess it's not worth it.

In a fit of annoyance, I submitted a PalmGear review and sent an e-mail to Wolfgang, the author, who wrote back a message this morning that was both polite and slightly confrontational. Granted, my original e-mail was confrontational as well, so maybe I'm at fault. However, both of us agree that we are not interested in pursuing any sort of ugly war. And my product isn't supported anymore, anyway.

But what are the "rules" about product naming, anyway? What if someone unknowingly uses a name you have already used? Do you have to have a trademark on the name in order to win a legal battle?