"Integration" has always been the "magical" word for the IT world. Traditionally "Integration" projects have always been after thought projects started after purchasing/building more than one system and result of making it to work together in some way for business to function with less frustration.
Complexity of IT systems and making it work together in the natural flow of day-to-day business process scenarios have always resulted in work around and half working systems. This is mainly because , integration has not been in the forefront of system designs.

I believe that integration should be a design / purchase consideration for business systems right from the very beginning.
Also Integration has always been thought as the middle tier level integration. I also think that the integration needs to happen at every level
• Data Level Integration
• System Integration
• Business / App Level Integration
• UI level Integration

It's fact that company of any size will have more than one system and without “integration” being in the forefront of the design, we would never achieve the full potential of the incredible power of software. Consider that a mid sized company in some form will have the below systems
1. Finance
2. HR
3. CRM
4. Sales Analysis
5. Service/Support System
6. Internet web site /e-Commerce
7. Business/Domain specific systems such as Inventory , Procurement, supply chain , professional services etc
8. Mail Infrastructure and Communication Platforms ( Live Meeting/Chat)
9. Planning/Scheduling
10. Intranet/Knowledge Management / Collaboration
11. Training/Social Networking ( blogs, newsgroups, discussion groups etc)
12. Business Intelligence
13. Productivity Tools
14. and many more...

And without well thought out integration strategy, the systems would never fit in the natural business/operation process of humans using the system  and allow them to get the most out of those systems.
 
With the advent of XML and web services, this dream is slowly coming into the forefront of system designs.

I hope, my friend Anand Rajagopalan ( co-blogger of this blog) who is working in the XML web services team in Microsoft  would be providing more insights into this topic in coming days.