Christopher Walker wrote a simple walk-through for building a form field to filter the contents of a DataView web part. As he said:

"The idea is to effectively create a simple "search" facility on a Data View web part, which is enabled by creating a Form Web Part with a single text field that provides this value as input to the Data View, the Data View is filtered based on the input value."

http://blogs.msdn.com/cjwalker/archive/2005/05/25/421980.aspx