I usually find this happens on any of the fast chargers - i.e. with the higher outputs. I've had it on my last few Nokia phones. I even had to dig out on older charger to useif I was sleeping, just so it didn't whine and squeel. It always sounds to me like it is trying to force the power in and constantly changes pitch and pulses as it charges (but tends to be more quiet when charging) - as soon as charging finishes, it makes a right racket!

However I don't believe it damages the phone - I can imagine the battery life may be affected from being charged so fast, but you would also assume that Nokia have built that into the battery/charging circuit designs.

I tend to be able to hear dog whistles - so this might not be as bad for some people! Pretty interesting though. The phone I currently use is an E65 (because my N900 has been sent away for repairs!) - and it whines like crazy with the charger that came with it!