This is a complete guess but you could try it anyway...
There is a file called MsDtsSrvr.ini.xml that defines the SQL Server instance to which the SSIS Service is "attached" (for want of a better word).
Perhaps your domain admin account has permissions to connect to SQL Server but the domain user acocunt does not.
Just a guess but try it anyway.
-Jamie
AHA. Yes, you're assuming too much. You have to tell SSIS Service which SQL Server instance it is "attached" to. Look in the XML file that I pointed you to before....you'll find an entry for server name which you should change to contain the named instance.
On my machine the XML file is located at C:\Program Files\Microsoft SQL Server\90\DTS\Binn
Kirk has a post on this here: http://sqljunkies.com/WebLog/knight_reign/archive/2005/06/08/15765.aspx
-Jamie
Additional info (from http://msdn2.microsoft.com/en-us/library/ms143504.aspx):
Instance-unaware services are shared among all installed SQL Server instances; they are not associated with a specific instance, are installed only once, and cannot be installed side-by-side. Instance-unaware services in SQL Server 2005 include:
Paul
Did you get to the bottom of this problem I ask because we are experiencing exactly the same thing here.
Thanks
Rachel
The solution to my issue was to reinstall the service pack 2.