If I create an application where all my data access is via the BO using the dll in windows forms. Can I deploy the same BO.dll to a website and access the backend data through the BO.dll ?

Yes. I have done this several times, will work fine. A BO can be used in either a WinForm and web application with no changes, the only change is how you implement the BO's, plumb their datasource, etc.

I have a few website's where the data import functions for a website are WinForms (simply because I am more proficient with Winforms) and the website uses the same BO's to display the data.

Hope that helps.

Keith Chisarik

Keith is correct.  This is the beauty of the BOs.  They can be reused and used on different platforms.  We do this in our CRM, medical application, etc.  It is a wonderful feature and benefit of BOs!