Actually, the GP requirements list Workgroup Edition as supported. The only differences seem to be in some of the backend DBA-type analysis services and Workgroup is limited to 2 processors instead of 4 with Standard. Otherwise, they both can only have a 1TB database and can only address 2GB of RAM.
I just don't know enough about SQL Server to ascertain if not having some of those analysis services would impair Great Plains. My conversations with a consultant concluded that they don't. But, the licensing is tricky with SQL Server Workgroup Edition, in that the CALs can only be used for the Workgroup Edition. In the event you needed to implement Standard for whatever reason, you wouldn't be able to reuse the CALs.
I think we're opting for Standard just to limit potential surprises down the road, although I'm relatively certain Workgroup suffices in a small business atmosphere where no DBA is present and you just need to get Great Plains deployed.
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Space travel's in my blood.