Just food for thought, but it is possible to get a Mac completely bent out of shape by running it hard with insufficient RAM. Apples default configs tend to be rather stingy, IMHO. Doubling what you get with the machine will always improve performance, and in my experience, will make lockups less frequent.
Also, the "Your system must be powered off. Press the power button for several seconds or press the reset button..." thing can be caused by untidily pulling devices out - an external firewire disk, for example. Is it possible you did that?
Regardless of the above, I'd have done the same and taken it back if that happened the first day I got it.