The unique structure of the IPCC includes both scientific and governmental review, but the input of diplomats to the final report is designed to be distinct and different from that of scientists.
Scientists who are experts in their subject matter prepare the chapters that go into the full assessment reports. Those chapters are scrutinized by individual scientists and scientist panels, whose questions must be addressed before a chapter can be approved for inclusion. The chapters making up the 2007 draft report from IPCC Working Group 1 report run to over 1,600 pages.
It is only the Summary for Policymakers, typically around 30 pages, that receives a word-by-word review, during the final plenary session, by diplomats from almost every nation in the world. The lead authors of the report are on hand at the plenary to make sure that any changes are scientifically valid. The diplomats have a say in how the Summary for Policymakers is worded, but the scientists have the last word on what is said.
The result of the IPCC process is a report that carries the weight of formal approval by the world's governments as well as the authority of hundreds of participating scientists. For more about the process, see the IPCC Fact Sheet: How does the IPCC review process work? (PDF).