Upgrade from Samba 3.0.24-r3 to 3.0.27 causes problems with Windows.

So after upgrading from 3.0.24-r3 to net-fs/samba-3.0.27, I immediately ran into problems with my mapped drives. Clicking on the drives from Windows, I received the following:

Local Disk (C:)
X:\ refers to a location that is unavailable. It could be on a hard drive on this computer, or on a network. Check to make sure that the disk is properly inserted, or that you are connected to the Internet or your network, and then try again. If it still cannot be located, the information might have been moved to a different location.

As always, my knee-jerk reaction is to hit the Gentoo forum, and I found this useful thread 'Samba 3.0.26 upgrade: home directories not working anymore'. Here a poster provided this great clue:

Changes to MS-DFS Root Share Behavior

Please be aware that the initial value for the "msdfs root" share
parameter was changed in the 3.0.25 release series and that this
option is now disabled by default. Windows clients frequently require
a reboot in order to clear any cached information about MS-DFS
root shares on a server and you may experience failures accessing
file services on Samba 3.0.25 servers until the client reboot
is performed. Alternately, you may explicitly re-enable the
parameter in smb.conf. Please refer to the smb.conf(5) man page
for more details.

Basically, Window clients need to reboot to clear the MS-DFS cache. I can confirm that a reboot fixed my problem.

Also, if you are using CIFS mounts, you'll now need to emerge net-fs/mount-cifs.