I found this thread today (http://forums.gentoo.org/viewtopic.php?t=28546) and noticed the script posted for mounting the tmp directories as tmpfs. Definitely practical and elegant, but I would suggest editing the directory names to something like /.fasttmp/maintmp/vartmp ... just on the _very_ _very_ remote possibility that a file would be written to /tmp under a subdirectory named var and then matches up with a file in /var/tmp. I know it's really obscure, but hey - you never know, right?
I posted an easy fix to this in the thread, but I really see it as much of a problem.
I think this is an personal issue. I just mount /var/tmp/portage as tmpfs, and be done with it.
Like I said, personal preference.