A weird thing happened.. when logging into my ESXi 4.1 box, my latest Windows Active Directory password wasn't working. So I tried my previous password and it worked?!?
I may have missed a small piece that needs to be configured.
Open vSphere Client -> Log in with your account -> Click the Configuration Tab ->
Click Authentication Services -> Click Properties in the upper right hand corner
Select Active Directory as the Directory Service Type
Type in your Domain and click the button below
Enter your username and password
All the boxes should update and be greyed out once it is joined. Close the client and reopen it using your most recent password.
It should log you in successfully now.