In an article over at the CreditMattersBlog, it appears that not only is Citi on a wild spree of canceling unused lines of credit, they are also lowering credit lines for accounts in good standing. What if this happens to you? Just call them and complain. There's a chance that you're previous limit will be reinstated.
http://www.creditmattersblog.com/2009/02/citibank-hacking-lines-of-credit-again.html