I'm also using geoserver 1.2.4 under JBoss 4.0.1 (maybe it's not sp1)
and I never had such an error (I had others...).
In any case it should be caused by having two different copies of
GeoTools (or any other class or JAR) visible inside JBoss.
Even if the two copies are identical this may cause problems, since
JBoss' class loaders space is shared between all the deployed apps.
If you're developing some other application that "uses" or
"is used" by GeoServer, for example a .SAR service, you should
undeploy and redeploy GeoServer each time you undeploy and redeploy
your own app.
If everything else fails, try to restart JBoss, sometimes it helps.
Also, I didn't remember exactly why, I had to stop using GeoServer
as an exploded .WAR directory and start using it as the "normal"
geoserver.war file, otherwise I had errors somewhere. This has the
inconvenient that you loose you configuration each time, so you have
to save the WEB-INF/catalog.xml file and the data/styles/ and
data/featureTypes/ directories.
Bye
Paolo Rizzi