Phil, I thought I had already tried that but apparently I didn't (or didn't do it correctly). I changed the connection string name from "My Connection String Name" to "My_Connection_String_Name". I suppose that worked because it seems like I got further than before but I'm still getting an error. Now I'm getting

Option "/CONNECTION My_Connection_String_Name;Data Source=Myserver;User ID=myuserid;pwd=thepassword;Initial Catalog=MyCatalog;Provider=SQLNCLI.1;Persist Security Info=True;Auto Translate=False;" is not valid. Process Exit Code 6. The step failed.

And again, here is the original string example (with the connection string name change):

"C:\Program Files (x86)\Microsoft SQL Server\90\DTS\Binn\DTExec.exe" /FILE "D:\MyFile\MyPackage.dtsx" /DECRYPT "mypassword"/CONNECTION

"My_ Connection_String_Name";"\"Data Source=MyServer;User ID=myuserid;pwd=thepassword;Initial Catalog=MyCatalog;Provider=SQLNCLI.1;Persist Security Info=True;Auto Translate=False;\"" /CONNECTION "MySecond.Connection.String";"\"Data Source=MyServer;User ID=myuserid;pwd=thepassword;Initial Catalog=mycatalog;Persist Security Info=True;\"" /MAXCONCURRENT " -1 " /CHECKPOINTING OFF /REPORTING EWCDI

I thought maybe the error was due to those double quotes at the end of the connection (\"" /) so I removed one of them but that didn't work. Or maybe it needs to be ";\"? I'm not sure. Again, this was generated from DTExecUI so my assumption would be that it should be correct (again, it worked when executing the package from DTExecUI).

Wow. They really don't give you much to go on when trying to debug this stuff.