From Lama release notes:
All spells cast from two-handed staves now use the wielder's caster level as well as calculate their DC's using the wielder's casting ability score, if they are better than the staff's default values.
http://forums.ddo.com/showthread.php?t=206066

a) What happens for multiclass characters with two (or more) different caster levels?
b) What happens if a cleric uses a magic missile clickie? Is cleric's caster level used?