Hi,
My quick thoughts on this-
For a tail current source of diff-pair, you need a certain min common-mode voltage on the gate of the diff pair to keep the tail nMOS in saturation(even if it's at the edge).
Sometimes, this isn't possible(even weak inv.) and you have to settle for a resistor in its place.
But, if you have sufficient Vgs, then, you can design to put your MOS into weak, moderate or strong inversion. Check out :
http://www.designers-guide.org/Forum/YaBB.pl?num=1168276093/4#4But, that post talks about getting 'gm' in different levels of inversion.
Is there a similar correlation for 'ro' whwich is what is required of a tail current source?