My skype was running. But suddenly won't start with the following message on console:

8/10/09 11:27:48 PM	[0x0-0x5b75b7].com.skype.skype[21885]	Check 2 failed. Can't run Skype

Delete cache/plist files won't work. You have to download and install Skype again. Remove Skype.app, download again from skype.com, drag it into Applications again, then it works again.

I googled around, if the console says "Check 1 failed", you may also need to reinstall it.