Exchange Server 2007 permits you to install the product and use it for 120 days in trial mode without entering a product key. If you intend to use your system in production you should enter your Exchange Server 2007 product key within that 120 day period (preferably before your system goes into production).

You can check the status of your servers using the Exchange Management Console.

[PS] C:\>Get-ExchangeServer | fl name,productkey,*trial* 

Name                              : MAILSRV
IsExchange2007TrialEdition        : False
IsExpiredExchange2007TrialEdition : False
RemainingTrialPeriod              : 00:00:00 

Name                              : MAILSRV1
IsExchange2007TrialEdition        : True
IsExpiredExchange2007TrialEdition : False
RemainingTrialPeriod              : 119.06:14:51.6643750

To add your product key to a server use the Set-ExchangeServer cmdlet.

[PS] C:\>Set-ExchangeServer -Identity MAILSRV1
-ProductKey 12345-ABCDE-12345-ABCDE-12345 

WARNING: The product key has been validated and the
product ID has been successfully created.
Note: This change will not be complete until the
Microsoft Exchange Information Store service has
been restarted. 

[PS] C:\>restart-service MSExchangeIS
WARNING: Waiting for service 'Microsoft Exchange
Information Store (MSExchangeIS)' to finish
stopping...

You can now use Get-ExchangeServer to verify the server is correctly licensed.

[PS] C:\>Get-ExchangeServer | fl name,productkey,*trial* 

Name                              : MAILSRV
IsExchange2007TrialEdition        : False
IsExpiredExchange2007TrialEdition : False
RemainingTrialPeriod              : 00:00:00 

Name                              : MAILSRV1
IsExchange2007TrialEdition        : False
IsExpiredExchange2007TrialEdition : False
RemainingTrialPeriod              : 00:00:00