base64 is included in coreutils as of 6.1, ad coreutils rightly blocks base64 ( bug 146149 ). However base64 does not block coreutils back, allowing the awkward situation of installing base64, clobbering coreutils' version, then getting blocks when trying to do anything.
IMHO base64 can be removed completely, unless some arch/userland doesn't have coreutils. Nothing seems to depend on net-mail/base64 Thoughts, anyone?
OK, net-mail/base64 has been package.mask-ed, and will be removed soon, unless someone speaks up. Thanks for reporting.