This is a feature that is frequently requested and is being implemented, I believe, for SAS 9.2 and the BI Platform built on SAS 9.2.
Dynamic and cascading parameters can be achieved, however, if you write your own custom front end to execute stored processes -- using .NET or Java or DHTML or HTML forms or .... . When I have students who really need dynamic or cascading parameters, they generally write their own front end code.
cynthia