I seem to have been asked this a number of times recently, so here is the answer. To take a public folder offline, simply add it to your favourites and then enable synching of public folders. Mark the public folder as a favourite (right click on the folder and select make favourite Go into your account...
I was recently asked by someone how to do this. Considering the fact that there are pre-defined public folders in SBS, including those for contacts I thought it was worth sharing this with a wider audience. Set this up for a customer using SBS and they can easily share contacts across the whole business...