OT: Copyright Infringement

I got a Myspace message from some guy (who I don't know) regarding a
new song. (Memorial Drive) The heading was "Copyright Infringe" and he
said the song was a rip of Piano Man, either through "marketing
strategy or ignorance", his words. I usually don't put much stock in
legal advice from 47 year old masons who call me "Dude" but it got me
thinking.... 

The song does have a similar walk down progression at the beginning,
as does Mr. Bojangles and probably a dozen other songs. I certainly
don't want get in any trouble, especially if it makes me look like a
Billy Joel fan. I pretty much blew his message off but I'm just
wondering if anyone knows of any standard for determining this type of
thing...not specifically with this song but in general. 

More im****tant, this doesn't mean I'm gonna have to learn more chords
does it? 

G.