I can feel a slight tingling whenever I touch the chassis of my powermac G5. I have tried to plug it in in different outlets and surge protectors and the same thing happens. Since the warranty is long gone will Apple do something about this?
Apple will probably do nothing, you can always try though, if it has a leaking cooling system (dual 2.5 june 2004) (dual 2.7 early 2005) (quad 2.5 late 2005) you might have a little better chance of Apple listening, some have had them replaced with mac pros, some had them repaired, some Apple tells to go fly a kite and buy a new imac. As of last year a few have had sucess although that window gets smaller everyday.
If its not liquid cooled you probably have next to no chance of them even talking to you, since any connection bridging to the case was not caused by the safety hazard that liquid poses around electronics. They'd probably just push the blame to anyone but them. You never know though, find out what exact model you have, and try to phone tree through to Apple Consumer Relations and tell them you have a safety/health hazard concern because you continue to get shocked when touching the case. Who knows, maybe they'll value your business and stand behind there product, maybe not.
Aside from that all you can do is learn how to take it apart and seeing what is bridging a connection somewhere, should be able to see it in the dark, could be a screw that fell into the power supply. And then replace whatever is acting up, either the logic board or the power supply or whatever it is.
I have this happen to me all the time with my G5 at work and with my PowerBooks. It's static electricity (in my case at least). I have taken to touching a metal file cabinet or something metal before I touch my PowerBooks or G5.