Summary says it all. If you choose to install mail-mta/exim then setup bombs out saying that it can't emerge mail-mta/ssmtp. A bad screenshot of the problem can be viewed here: http://img153.imageshack.us/my.php?image=15bj3.jpg
The error is slightly misleading. The portage output is being parsed wrong by the installer and isn't seeing the blocker correctly. As a workaround, you can add 'mailwrapper' to USE so that there isn't a blocker. *** This bug has been marked as a duplicate of 125699 ***