I had the same problem and Andrew is correct. Check your classpath variable "M2_REPO". It probably points to an invalid location of your local maven repo.
In my case I was using mvn eclipse:eclipse
on the command line and this plugin was setting the M2_REPO classpath variable. Eclipse couldn't find my maven settings.xml in my home directory and as a result was incorrectly the M2_REPO classpath variable. My solution was to restart eclipse and it picked up my settings.xml and removed the red exclamation on my projects.
I got some more information from this guy: http://www.mkyong.com/maven/how-to-configure-m2_repo-variable-in-eclipse-ide/