I was having this problem:
D:\progetti\geoserver\trunk\web>mvn jetty:run
[INFO] Scanning for projects...
[INFO] Searching repository for plugin with prefix: 'jetty'.
[INFO] artifact org.codehaus.mojo:jetty-maven-plugin: checking for updates from central
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD ERROR
[INFO] ------------------------------------------------------------------------
[INFO] The plugin 'org.codehaus.mojo:jetty-maven-plugin' does not exist or no valid version could be found
[INFO] ------------------------------------------------------------------------
[INFO] For more information, run Maven with the -e switch
[INFO] ------------------------------------------------------------------------
[INFO] Total time: < 1 second
[INFO] Finished at: Sat Sep 09 16:05:19 CEST 2006
[INFO] Final Memory: 1M/3M
[INFO] ------------------------------------------------------------------------
This is due to a clash between mojo and mortbay plugins:
http://jira.codehaus.org/browse/MJETTY-2
It hit me because I do have a checkout of the whole mojo projected that I build from
time to time.
The only solution I found is to completely wipe out from your repository
the org/codehaus/mojo directory. After that, plugins gets downloaded again
and all works fine.
Cheers
Andrea Aime