I also received an email from my ISP. Should I be concerned about legal action or is this more of a notification/warning?
Comcast has received a notification by a copyright owner, or its authorized
agent, reporting an alleged infringement of one or more copyrighted works made
on or over Comcast's High-Speed Internet service (the 'Service'). The copyright
owner has identified the Internet Protocol ('IP') address associated with your
Service account at the time as the source of the infringing works. The works
identified by the copyright owner in its notification are listed below. Comcast
reminds you that use of the Service (or any part of the Service) in any manner
that constitutes an infringement of any copyrighted work is a violation of
Comcast's Acceptable Use Policy and may result in the suspension or termination
of your Service account.
For more information regarding Comcast's copyright infringement policy,
procedures, and contact information, please read our Acceptable Use Policy by
clicking on the Terms of Service link at http://www.comcast.net.

Sincerely,
Comcast Legal Response Center

Copyright work(s) identified in the notification of claimed infringement:

Title: ******************

Infringement Source: Gnutella
Initial Infringement Timestamp: ** Jul 2007 ********** GMT
Recent Infringement Timestamp: ** Jul 2007 ********** GMT
Infringer Username:
Infringing Filename: ********************.mpg
Infringing File size: *****************

Just stop sharing the file.