if you are going to go the whole hog, then I vote data warehouse. As I said in another post there is no standard for federation and you have to build all the integration yourself. Might as well extract, transform and load so you are technology independant.
But there are easier options:
Buy a service support tool (usually mis-named a "Service Desk" by the vendors) that properly integrates service desk, incident, problem, change, asset and config (and hopefully service level reporting) in one tool with one set of data (not all of them do). Now you have 90% of your CMDB in one place. if your network and systems management data is somewhere else, big deal. You'll cope. You can never get 100% of data in one place anyway. So what if it is 90% or 70% or 30%: you'll still need the same processes to cope with the fact that it is not 100%. I call this
the CMDB boundary problem.
Then have two people who are your walking CMDB for impact analysis. They understand the data, or they know where to look, to work out what it means when a component fails or a RFC wants to change a CI. The vendors will sell you automated impact analysis tools, but these tools can be notoriously stupid in their deductions. They only have to get it spectacularly wrong once and everyone will go validate each automated impact deduction with the "human CMDB" anyway.
(But don't buy any tool until you have your processes sufficiently designed to know what you need,and you have enough Change process implemented to protect the data. Without adequate Change in place your data is crap minutes, hours or days after you load it.)