Lately, Some of my buddies and I have been experiencing problems opening applications we downloaded from the web (most of these expanded from a .sit archive). The app would show up in the dock for less than a second then disappear. I had this same problem for a program called "Network Beacon" which I really wanted to work. So, for some reason I decided to check the permissions on the app. Inside Network Beacon.app, I noticed that the file located in the Package Contents -> MacOS folder was not executable at all. A simple jump to the terminal and and typing this:
chmod +x /Applications/Network\ Beacon\ 1.0.1/Network\ Beacon.app/Contents/MacOS/Network\ Beacon
cleared up the problem and I was able to launch the app.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031208012323150