Automatic printing has been a frequently requested feature for Sparkbooth. I have been working on a solution which is waiting on the release of Adobe AIR 2.0. [Note: Please do not install Adobe AIR 2.0 beta and run Sparkbooth. I am getting reports of webcam and printer issues when running the beta. Only run Sparkbooth on the latest release, Adobe AIR 1.5.3]
In the meanwhile, wedding photographer Josh Rodriguez has come up with a solution in Mac OS X using a simple Automator script. Check out Josh's article. Great job Josh!
[Update April 2, 2010]: Someone has pointed out AutoPrint as a similar printing solution for Windows users. If you are using this, please let me know how well it works.