Integrating CRM with ERP/Financial systems can be a challenge. Particularly if the systems are from 2 different vendors, which is often the case when using Salesforce.com CRM.
At Facebook, we've gone through several iterations of integrating Salesforce with Oracle Financials and the team has arrived at a fairly stable and reliable integration process (kudos to Kumar, Suresh, Gopal, Trevor, and Sunil for making this all work).
Here is the basic flow (see diagram below):
1) The point at which Salesforce CRM needs to pass information to Oracle is typically once an Opportunity has been closed/won and an order or contract has been signed.
2) Salesforce is configured to send an outbound message containing the Opportunity ID to an enterprise service bus (ESB) that is configured to listen for specific SOAP messages from Salesforce.
3) The ESB accepts the outbound message (now technically an inbound message on the receiver side) and asserts any needed security policies, such as whitelist trusting the source of the message.
4) This is the interesting part. Because the Salesforce outbound message wizard only allows for the exporting of fields on a single object, the ESB must call back to retrieve additional information about the order; such as the Opportunity line items, Account, and Contacts associated with the Order.
In Enterprise Application Integration (EAI) speak, this is referred to as a Content Enrichment pattern.
5) An apex web service on Salesforce receives the enrichment request, queries all the additional order details, and returns a canonical XML message back to the ESB.
6) The ESB receives the enriched message and begins processing validation and de-duplication rules, then transforms the message into an object that can be consumed by Oracle.
7) The ESB then inserts the Order into Oracle.
8) The Oracle apps API inserts/updates the various physical tables for the order and throws any exceptions.