Mashups seem well suited for information centric / lightweight BI applications, but how do these solutions evolve over time or provide enterprise capabilities for inclusion of more complex business logic, or support for business processes, rules, event processing, etc.

Mashups are done on services. Business processes, rules engines and event processors can be exposed as web services and mashed pretty easily.  The reverse is also true: a mashup can be embedded in a larger business process, in support of automated decision-points in the process.