Well Maya 2008 has many bugs that I had to deal with so here are some of the fixes for MISSS_Fast_Skin Bump Missing and optimization flag fix and other things:
http://www.tgjay.com/htms/tutorial/mr4Maya9E.htm
Also you need to have a check box checked to do 32 bit displacements in Maya 2008. This one held me back for a day and was really annoying. You can do that by going here Window > Settings/Preferences > Settings > Rendering > Mentalray preferences > use maya-style alpha detection in file textures. That will make the displacements work properly. Don't forget to turn feature displacement off and to use your Mental ray approximation editor. For 32 bit Displacements Alpha Gain should be set to 2.2 and Alpha Offset to -1.1. This doesn't change for 32 bit displacements. However you can mess up your render if you scale your model because 32 bit displacements are scale sensitive.
This was all enough for me to go back to 8.5. Good luck if you have no choice but to work with Maya 2008.