IBM CIO Jeanette Horan told MIT's Technology Review that her company has banned Siri outright because it worries that the spoken queries might be stored somewhere.
Part of the problem is that Apple's iPhone Software License Agreement says that when you use Siri or Dictation, the things you say will be recorded and sent to Apple in order to convert what you say into text. Siri collects a bunch of other information such as the names of people from your address book and other unspecified user data, all to help it "do a better job".
Apple refuses to say how long it stores this information, and who gets to see it. But if you were dumb enough to press yes on the user agreement you consent to: "Apple and its subsidiaries' and agents' transmission, collection, maintenance, processing, and use of this information, including your voice input and User Data, to provide and improve Siri, Dictation, and other Apple products and services". The implication is that Apple can give your data to whoever it likes.
The American Civil Liberties Union warned that Siri users were signing their souls away but of course no one really listened, they just saw Siri was shiny, and besides, there was no other reason to buy the iPhone 4S.
It must be fairly bad for IBM to ban the software. IBM doesn't ban Google which has also been involved in similar privacy rows. Since Siri can be used to write e-mails or SMS, Apple could be storing confidential IBM messages.