Depends probably how "deep" you want to look: for me the possibilites of abstacting away functionality or to exchange wrappers/strategies are layers. so on the biz side I have a data access layer (for instance "enhancing" cursors used in grids later by adding related info in non-normalized way) and on the data side you might have a layer abstracting away the SQL/RI differences between backends.

On the GUI side you might have a validation layer, which in fat clients just call into biz validation, while on Web frontends simple validation rules might be handled in duplicated javascript code to minimize round trips, while the biz tier still has those rules for automated web service handling.

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>Maybe I'm just locked into an idea I have been familiar with for a while but I don't know why I would need more than a GUI layer, a data layer, and a business rules layer.
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>>Yes, but I can't list them because it is covered under an nda. I've actually seen more. Have you seen MSFT's latest recommendations?