Has anyone ever integrated an external web application with Siebel On-Premise CRM? Note that I'm not talking about Siebel On-Demand SaaS, but their behind-the-firewall product. Specifically, I'm trying to achieve two-way synchronization of CRM objects (contacts, accounts, sales opportunities) between my web application and a customer's internal Siebel setup. Are there any well-known techniques for initiating or receiving external connections from a Siebel On-Premise installation.
Siebel On-Premise offers a wealth of integration options. Start with Siebel Bookshelf and specifically Integration Platform Technologies: Siebel Enterprise Application Integration.
For your more static data, have a look at MDM. In this case refer to Siebel Master Data Applications Reference for Industry Applications.
Otherwise, webservices or Siebel's prebuilt ASI's offer alternatives. Exchanging XML through webservices which map data to Siebel Integration Objects, which map to Siebel Business Components, is pretty much standard fare from the architectural point-of-view.