Bumping because we would like this feature as well.
Our business use is we have a request to build a server for example, and we have multiple locations. We would only like to have one "Windows/Unix Server Build" ticket in the request catalog so for example, if the user selects a custom Location field or the predefined "Site" field, the ticket would route to either Location A, or Location B. Lets say the user selects location A, next we would like it to look at a custom OS Type field that would include Windows and Unix. Based on the users selection it would either route to "Location A: Unix Team" or "Location B: Windows Team" again, based on the selection of Location and OS type.
This function would only be available through incident requests, which we do not want to use since the Server Build Request is a service, not an incident.
I hope this is sufficient reasoning for business rules to apply to the Service Catalog in our use case.
Clearly this feature would help quite a few people and create some happy customers and could even be a MAJOR selling or breaking point for an organization to purchase the software.