As far as I know, there is still no tomcat-5.5* ebuild available.
You'll find this bug :
http://bugs.gentoo.org/show_bug.cgi?id=75224
and the discussion is open.
From there, I notice a place where there are some attempts for a working ebuild from the Experimental Java Tree :
https://gentooexperimental.org/svn/java ... perimental
I think I may try helping and do some stuff in order to make this ebuild working.
Right now, I'm still struggling :
COMPILING failed due to keyword 'enum', etc....