Here is a notification we received from Stampin' Up!:
Carry On Hostess Stamp Set Unavailable
June 5, 2012
Due to unforeseen legal issues, we have turned off the item numbers in OEX for the Carry On Hostess Stamp Set (126645, 126647, and 128156). As we are working through the details, please be aware that Demonstrator Support does not have any additional information at this time. We'll post a current update as soon as we know more.
Here is more information received from fellow demonstrator:
The stamp set is based on a British poster designed in war time. It was designed by the British Ministry of Information, which was eliminated after World War II. The copyright owner on these images would then be the British Government, except that due to Crown Copyright law, copyright on it has expired.
Since the poster resurfaced, and gone viral, a company registered Keep Calm and Carry On as a business name in the UK. They sell items make with that design as well as derivatives. Since it is their business name, the have been filing for trademark privileges. They were denied the trademark in the UK. They have a trademark in place filed with the European Union, however it is currently being disputed. They have since filed for trademark in the US and Canada.
So... to answer your question, based on the above and my knowledge of the issue, I'd say go ahead and use the set for your own personal use.
Here is a video on the history of the poster....interesting: