Other users have reported shock issues too.
Since you have a propensity for this. You said, "This has actually happened a couple of times before but not to this degree."
Next time you charge your iPhone, plug the USB cable into the iPhone and then set the iPhone down carefully. Then plug the cable into the charger or USB port of your computer. When done charging reverse the steps. First unplug the USB cable from the power source. Then unplug the cable from the iPhone.
Today I got the electric shock from my iPhone 3gs connected to my iMac. Exact same scenario for me too. I have felt minor static around the metal edges of iMac but did not bother about it much till date as I have seen similar thing on Macbook Pro as well.
Today, after connecting my iPhone, I just picked it up accidently touching the metal surface near the docking port. OMG, i did not realize what happened, I just dropped the phone on the table and shook my hands. Then realized what happened.
I was too scared to touch anything then and quickly grabbed a tester and placed it on the metal edge near the docking port (the cable remained connected) of my iPhone 3gs. The tester was glowing. I then I shut down the iMac (thanks to wireless keyboard and mouse). Tried it again, samething. Weird. Why would there be a voltage on the USB ports when the **** machine is shut down? This stayed on for nearly 5 mts. I checked all the connections, then plugged everything back again. Now it is gone. However, I am scared to touch iMac until Apple engineer walks down to my place and says everything is fine.
Definitely it is not a static and at the same time, it is not as bad as the ones from the power socket, but it is something serious to be taken up with Apple. I called them up and they have registered my case. The support engineer asked me a lot of questions:
1. Are there any injuries or property damage? I said No. (Good!)
2. Any cable burn outs, fumes or fire or any funny electric smell? I said No
3. How long was this static problem I faced (nearly two weeks or so)
4. How long this voltage problem stayed on (i said 5 mts and I kept testing it using the tester)
Finally he said, it would take 4 business days.
I also told them that now things seems a normal (I am typing it on my iMac with needless to say the tester is next to me). I will keep you guys posted on this.