Are federal officials allowed to say 'Merry Christmas'?

Newt Gingrich says the nation's obsession with being politically correct has come to this: Federal officials aren't allowed to say "Merry Christmas."
   
In a campaign appearance in Davenport, Iowa, Gingrich said that "no federal official at any level is currently allowed to say ‘Merry Christmas.’"

We found that while it’s true that the intersection of religion, the workplace, and the federal government is legally tricky due to issues surrounding the separation of church and state, it is ridiculously false to make the claim Gingrich did. Guidelines in force for the past 15 years give substantial freedom for personal religious expression in the federal workplace, and neither those guidelines nor federal law includes anything like an outright ban on a federal official saying, "Merry Christmas."
   
The closest Gingrich came to accuracy is that Congress does bar taxpayer-funded official mailings of all types of greeting cards. However, such rules affect only one class of federal employees (those who serve in Congress); the rules are only about postage (lawmakers are free to send cards on their own dime); and it certainly does not abridge the right of any Member of Congress or congressional employee to speak the words "Merry Christmas." We found examples where they did. We rated the claim Pants on Fire!