HOW TO: Implement Custom Parameters for DataSource Controls
Solution
Custom parameters for data sources
by Eilon Lipton (ControlBuilderFAQ)
Create your own Parameter to the data-source control's parameters collection
by Fredrik Normen
Accessing and Updating Data in ASP.NET 2.0: Creating Custom Parameter Controls
by Scott Mitchell (4GuysFromRolla.com) - Showcases a custom parameter which returns the current datetime and a custom parameter which returns the current logged on user ID.
Related Resources
MSDN: Tutorial 5: Declarative Parameters
by Scott Mitchell
MSDN: Data Source Controls, Part 2: Parameters
by Nikhil Kothari