We just had a look at the document provided by Apple to repair centers regarding the Mac mini 2010. They illustrate how hard Apple worked to make things even smaller to be able to pack all components in a even smaller enclosure, especially the motherboard.
The motherboard is now roughly the size of a CD. The PSU is really tinny even though it can deliver up to 85W. If changing the RAM modules would not be a problem, you will need to remove the fan and the motherboard to be able to reach the HD. We will provide more information as soon as we can get our hands on a unit.