The Tier I BO consultants have all told us that we should be using MS Sql Server. And I was in a phone conference with a VP at BO (prior to the SAP acquisition). He was promoting switching back-ends to MS Sql Server to my then boss who brought the BO horror show in here. As best I can recall, he asked, "How much market share does Microsoft Sql Server have? Which one do you think we're going to support first?" (Rhetorically of course). The only first-hand experience I have with BO and commercial databases are MS Sql Server and IBM DB2 UDB. We ended up using PostgreSQL for this because it actually worked better than IBM DB2 UDB after months of trying and DB2 being on the official "Supported" list.

I do know that Cleveland Hospital (an Oracle shop) dumps data to an MS Sql Server farm for their Business Objects stuff. I figure there's got to be a reason why the do that.