I've seen other designs on Threadless that featured song lyrics. I would think the song has long since come into the public domain by now. And if the admins at Threadless had any issues I would think they'd have said something.
Okay, so the song isn't public domain, ...apologies for the presumption (BTW, WarDrobe, yes I have heard Beatles songs in film and TV...mostly covers, anyhow). If it turns out there is a copyright issue, I'm sure someone will catch it before anybody's gotta call a lawyer.
This is my first attempt at a Threadless design. Mercy please? (I'm kidding, let me have it if you feel the need.)