Apparently it was purposely removed, according to this thread in the Server forums: http://social.technet.microsoft.com/Forums/en/winserverfiles/thread/dbfa011b-7c27-4e1d-b1a4-f0f8839b2d46
I understand that enabling administrator to set NTFS Security
permissions across multiple folders would be a good feture. However,
based on our experience, there are a number of users accidently mess
up ACLs on massive selection files and folders. In Windows vista and
Windows server 2008, To preventing this from occuring, it was designed
to remove the Securtiy tab when multi-selection on folders. Currently,
you can use icacls to grant NTFS permission on massive folders and
files. Sorry for the inconvienence to you.
Meanwhile, I will forward your feedback to our product team to see if
they will have a replacement of this functionality in the future.
Thanks for your understanding.
Needless to say, this response has been greeted with derision by various others in that thread. Now maybe Nick's comment to the question basically highlights Microsoft's rationale, but tbh it's a pain in the neck for those admins that do know what they're doing...
A workaround (also described in the thread) is to access the server from a 2k3 or XP client, then permissions can still be modified directly through the GUI.