Finally, I could induce myself to post this issue: On desktop systems, /usr probably resides on a single, writable mounted partition with the rest of the filesystem. On server systems, it is likely that /usr is swapped out to a separate, read-only mounted partition. This causes portage no longer to be allowed to write to /usr, unless the administrator remounts it rw, before syncing f.e., and then again remount it ro. I therefore suggest to change the default value for PORTDIR to /var/portage instead of /usr/portage.
If you really need to mount /usr ro, then you can have a dedicated partition for /usr/portage mounted rw, no?
I don't even want to imagine how many systems that would break. Personally I've moved it to /srv/portage/porttree, but for the general public I'd push this back until there are real changes to the repo structure warranting this.
