You can do this in Outlook 2007 by creating a custom appointment form, and then using that for your appointments. Good instructions are found at
http://www.liquidstate.net/2007/12/1...-outlook-2007/
Even though the instructions tell how to set an appointment to be "Private" by default by changing the "Sensitivity" value, the field right below it in the list is "Show Time As". There, type in "Free", publish your form, etc. (continue following the directions in the link), and you are good to go!
Hope this helps.