One of the new features added in Exchange Server 2010 Service Pack 1 was called Shadow Redundancy Promotion.
Service Pack 1 creates a setting in EdgeTransport.exe.config to control the behaviour of this setting, and it defaults to a value of “False”, so that the feature is turned off.
Enabling Shadow Redundancy Promotion is done by setting the ShadowRedundancyPromotionEnabled setting to “True” in EdgeTransport.exe.config (found in c:\program files\microsoft\exchange server\v14\bin) and restarting the MSExchangeTransport service.
When you install Service Pack 2 for Exchange Server 2010 it resets this value to false and therefore disables this setting. This is not documented (at the time of writing) in the read me for the service pack.
This does mean of course that if you have chosen to enable Shadow Redundancy within your SP1 Exchange 2010 organization, this will get turned off again when you apply the service pack. Note that other settings in EdgeTransport.exe.config are not reset on applying the service pack.
When the service pack resets the value, it does make a comment in the config file to sort of indicate what it has done. It notes the following text above the line that disables Shadow Redundancy.
<!–Exchange task Set-AppConfigValue updated value for dictionary key ShadowRedundancyPromotionEnabled from True to False at 12/5/2011 5:09:54 AM—>
It does not say it has turned your configuration off again, but that is the effect of the change that the service pack installation makes.