This is something that I've wondered about before. I had a small interview with some people regarding some work, and the idea came up again. So, I've been thinking about it.
The issue is having two databases. One as an OLTP, and the other as a Data Warehouse. Being I worked with Oracle, and I like a good deal of the Oracle way of doing things, I am going along with Oracle terms. Basically an OLTP is an On-Line Transaction Processing. It is a database that is designed more for input than for output, and most queries are small. A good example would be an ATM machine. In such a case, the database should be normalized, for authoratative answers, and less work on input.
On the hand, a Data Warehourse is made more for output than for input, and most queries are large. A good example would be a data mining firm. In such a case the database should not be normalized, for quick answers, and less work on output.
The problem is, many companies need both. There is always information going in, and every day a number of people want some serious reports. That's when the normalization fights come in, and the line in between OLTP and Data Warehouse blurs beyond recognition.
What I've been wondering, is why not have both? The OLTP could be normalized, and be the authoritative database, and the Data Warehouse could be updated nightly (or weekly). The only loss really is the reports are woking on data from yesterday. But that shouldn't be much of a problem.