emerge --regen won't run if user isn't root, this is a bug- nothing in that code path should require root (it's no different then what emerge -s does).
...affects 2.1 at the very least, prolly affects 2.0 also.
In svn r3346 --metadata and -regen are now allowed for users in the portage group.
This has been released in 2.1_rc2-r2.