There are some important checkbox fields in salesforce.com that you may find useful to reference in formulas.  "Do not call" and "Email opt out" on the Contacts tab are but a couple examples.

Is this possible, you ask?  Yes, but there's a trick to it: checkbox fields are treated as Booleans with a value of TRUE for checked and FALSE for unchecked.

You can use a checkbox field directly in an IF function as follows:

IF ({!HasOptedOutOfEmail} , "checkbox is checked", "checkbox is unchecked" )

Note that if you compare a checkbox field to the literal TRUE or FALSE in an IF statement, you will get the following syntax error.

IF ({!HasOptedOutOfEmail} = TRUE , "checkbox is checked", "checkbox is unchecked" )
Error: Incorrect argument type for function =.