When installing rails:3.0, "eselect rails" doesn't work as it isn't able to deal with rails-3. This problem was fixed in 'app-admin/eselect-rails-0.16', but 'dev-ruby/rails-3.0.3' does only depend on 0.15. The attached patch fixes this.
Created attachment 259617 [details, diff] Updated eselect-rails dependency for rails-3.0.3 ebuild
Silly oversight on my part. :-( Now fixed in CVS, thanks for the report.