I have seen questions like Installing Exchange 2010 Standard on SBS 2008 Standard and inplace upgrade of Exchane 2007 to Exchange 2010 in SBS 2008 etc on few forums. You will find the answer for why the inplace upgrade and the exchange 2010 installation can not be done on SBS 2008.
SBS 2008 comes with exchange 2007 and the exchange 2010 having a different schema structure, It will not allow you for an inplace uppgrade as well. See the operating system requirement for installing Exchange 2010 on the below article.
The exchange 2010 can be installed succesfully on 64-bit edition of Windows Server 2008 Standard or Enterprise with Service Pack 2 and 64-bit edition of Windows Server 2008 R2 Standard or Enterprise.
Hope you will not have confusion now.