I updated last night to iOS6 on the ipad3. No SIRI under settings either.
I notice I'm also missing PASSBOOK too. The new clock and maps are there, so I know it worked.
I also tried the "restrictions" tip within this discussion . . nothing.
I've done full restore to different backups, nothing . . .
My update took place by connecting directly to my iMac. Imac had latest OSx and Itunes.
I wonder if that may be part of the problem.
My friend got SIRI update fine, but did wirelessly.
I have a call with Apple Tech scheduled this morning. If there is a solution, I'll post it . . .