On 10g, Instanceconfig.xml is THE parameter file of the BI Presentation server. With the advent of 11g, instanceconfig.xml is also the configuration file of Oracle BI Scheduler
The instanceconfig.xml file stores the configuration settings that affect Oracle BI Presentation Services.
You will find it in this directory :
The version 11g permits now also to change some configuration with FMW control > Farm_Bi_Foundation > Business Intelligence > Coreapplication.
For Oracle BI Scheduler, the file is located in this directory : ORACLE_INSTANCE\config\OracleBISchedulerComponent\coreapplication_obischn