Nick Malik has prompted a great discussion on the difference between accuracy in architecture and IT. He asks why IT architects don't produce blueprints that are as accurate as those produced by architects in the traditional world of physical construction.
The kind of accuracy Nick describes in traditional architecture is about quantity. The costs of a building are largely determined by the physical dimensions. (The cost of the carpet depends on the floor area.) So the first person who looks at the blueprint is not the builder but the quantity surveyor. The blueprint has to be good enough to enable reasonably accurate cost estimation.
We don't usually do that in IT. There is no How-Many/How-Much column in the Zachman framework. You can't work out quantities from a UML diagram. In a pre-SOA world, we thought cost estimation was largely about counting the number of components to be constructed (simple, medium, complex) and putting them into a time/effort formula. But this approach to cost-estimation is increasingly irrelevant to SOA.
If you are only building a garden shed then you possibly don't need a professional architect or surveyor. If you are building a tower block then you certainly do. The people who are doing serious architecture in an SOA world are those operating at internet scale - for example redesigning Skype so that it doesn't fall over on Patch Tuesday (see Skype Skuppered).