Can a power MOSFET for switching application be used as a linear amplifier?
I had a similar question.  From reading application notes and presentation slides by companies like International Rectifier, Zetex, IXYS : 


The trick is in the heat transfer.  In the linear region, a MOSFET will be dissipating more heat.  The MOSFETs made for linear region are designed to have better heat transfer.
MOSFET for a linear region could live with higher gate capacitance


IXYS app note IXAN0068 (magazine article version)
Fairchild app note AN-4161