If ACCEPT_KEYWORDS is not set in /etc/make.conf g-cpan-0.15_rc2 creates ebuilds containing: KEYWORDS="ACCEPT_KEYWORDS"
bah. this was fixed at one point to fall back to the list in /usr/portage/profiles/archs. I'll check to see why that isn't the case. In the short term, if you create a .gcpanrc in the user's home dir you can set it to anything you want there. Not a fix, I know, but it would get you past the hump.
Fixed in rc3. I'll post it to portage in the next day or so (want to get one more bug fix in before rc3 is up). Thanks!