I know what you mean, I have had to do this alot. Here is a what I do at work and the employee will never know.
Server Side:
Go to start>administrative tools>Active Directory Users and Computers
Go to your users and right click on the user in question
Select Properties
Go to the Exchange Advanced Tab and select Mailbox Rights
Click on Add and add yourself.
In the Permissions box below add, give yourself every permission except the special permissions and external account.
Click Apply, then ok.
Outlook Side:(2003 & 2007)
Go to tools and select Email Accounts/ Account Settings
Select view/change.
Select More Setting and select the Advanced Tab
Select Add and Type in the user in question.
His mailbox is now in your outlook program.
Again the employee will never KNOW, ever. You have the right to do what you feel is right, YOU run the company.