Not sure if you updated Java recently but it's caused problems for several people. Your answer might be re-installing Java:
Apple - Downloads - Application Updates - Java for Mac OS X 10.5 Update 4 & then do a Safe reboot & repair permissions then reboot normally.
How to do a Safe Boot? = Hold down SHIFT key whilst Booting up until you see the apple icon. You will then be required to login. Open Disk Utility found in Utilities folder & repair permissions on OSX. Then reboot normally.
If still problems install the beta version of LW:
LimeWire 5.2.2 (Beta) - Lime Wire (if you are using LW 5 I mean.)