Alpha5: If you are still having this problem & haven’t solved it yet, I would follow Arachne’s advise. It sounds to me like the Power Supply is fried. I’ve seen this type of symptom occur numerous times.
In your original post, you said that the problem occured after you plugged the power cord into the PSU while the power switch was on. This could have caused a small power surge, but just enough to damage the PSU. The reason you are seeing the case light flicker is because a miniscule amout of electricty is still flowing through the unit, just enough to flicker the LED lights which require very little power.
The other worst case scenerio would be is that you fried your motherboard. This can happen with power surges.
If you could get your hands on a power supply tester (you can buy them at minimal cost at just about any electronics store) you could test the PSU. The other way you could test it is, remove the PSU from the case & plug it into an electrical outlet. If the PSU fan(s) spin, then most likely it’s the mobo. If they don’t spin, then it’s the PSU.