Just thought I would post this much after the fact to get the info out in webspace for when I forget it next time...
Had the same problem setting up a Linked Server for Informix:
Cannot create an instance of OLE DB provider "Ifxoledbc" for linked server [...]
The solution was to follow the advice scattered about out there:
1. In SQL Server Manager, go to the Provider folder under the Server Objects folder and set the "Allow inprocess" box on the appropriate driver's properties
2. On the Server machine invoke the Component Services located at the Control Panel/Administrative:
• Expand Computer Services/Computers/My Computer/DCOM Config.
• Locate MSDAINITIALIZE and Right-Click on it.
• Click on the Security Tab.
• In the "Launch and Activation Permissions" click on "Customize", then click Edit.
• Add the users or appropriate group and check all permissions (Local Launch, Remote Launch, Local Activation, Remote Activation).