One to watch out for when upgrading to Exchange 2010 SP1 is not to include a rollup in the updates folder of the extracted SP1 media, as it will cause the upgrade to fail any you will notice the following errors in the ExchangeSetup.log
Error in the Exchangesetuplogs
[ERROR] Installing product D:SupportEXCHANGE 2010 SP1 + R2exchangeserver.msi failed. Fatal error during installation. Error code is 1603.
[ERROR] Fatal error during installation
Ending processing install-msipackage
The Exchange Server setup operation didn’t complete. More details can be found in ExchangeSetup.log located in the <SystemDrive>:ExchangeSetupLogs folder.
End of Setup
After Running the upgrade for the third time you will then see the below in the log.
[REQUIRED] The previous installation path could not be found in the registry. Only disaster recovery mode is available.
[REQUIRED] The previously installed version could not be determined from the registry. Only disaster recovery mode is available.
So just be careful not to put any exchange rollups in the update folder during your SP1 upgrade !