Hey abatareika did you have any luck with this in the end with the other solution you mentioned?
I have the exact same issue. Imac 27inch bought in September 2012. Looks like I may have had a powercut (unsure) but i came back to my flat to find the imac turned off completely.
The issue is that the button no longer works in the same way it used to. Previously when pressing the button I recall (maybe something with a working button could confirm) you get a slight soft 'click' as if the button feels like it is being pressed in. Now it remains extremely hard and pressing the button is very stiff and fails to make the same click, as if the button is somehow jammed.
It seems many many people have had similar issues to this and the apple support threads report this also.
Since my unit is 2 years old I am claiming EU consumer law to get it fixed, but annoyingly I now live in Sweden, Apple basically told me despite them having shops right down the road from me I would have to send the entire unit in, have someone in the UK walk into a store in a genuis bar, and can give me no guarentees that it will be covered or not.
Its a bit of a joke. For such an expensive bit of kit you think Apple would at least publically announce that this is clearly a hardware fault.
Does anyone else have any experiences of this?