Let me just say, I am loving SAS 9.2 and it's functionality for cascading prompts. I created a dynamic list prompt (the icing to that cake by the way) pointing to a sas dataset that has two variables: County and City. In Prompt #1, I created a County Selection pointing to this sas dataset.

In Prompt #2, I created a City Selection pointing to the same data set and moved to the 'Dependencies' tab.

All I had to do was select the County Selection prompt and the type of join with this selection (obviously an 'equal' join) and drum roll please, I had two prompts where the later (City) was dynamically built off the prior (County) selection. This is incredible! Out of the box! Amazing! (Can you tell yet that I love love love this!) Thanks SAS! So many development hours were spent in the prior version to create custom web stored process applications to do this, and now it just takes minutes!

References
In Stored Processes: (via Management Console, but STP Wizard in EG is similar)
http://support.sas.com/resources/papers/sgf09/355-2009.pdf
In Information Maps: http://support.sas.com/documentation/cdl/en/imsgs/61225/HTML/default/p06j16ojo5j2cjn1n3nb0uoqzzh4.htm
In Web Report Studio:
http://support.sas.com/documentation/cdl/en/citug/61084/HTML/default/n1lf65fbkh9hjkn104px61t8zivn.htm