Sadly there is no keyboard shortcut, yet.
The most recently minimised window will always appear at the far right end of the Dock (if you have it at the bottom of the screen), next to the waste disposal ornament.
Edit: In Mac OS X, keyboard shorcuts for an individual application can be overridden. You may find it handy to remap Entourage so that Command + M checks for new Mail.
[1] Open System Preferences and Type: “Keyboard shortcuts” in search and choose the first suggestion.
[2] Then click the plus (+) at the bottom of the list.
[3] From the Application menu, pick Entourage, if it is not there, choose Other... from the bottom of the list and locate it.
[4] Type in the Menu Title exactly as it appears (e.g. “Send and Receive Mail)
[5] Click once the keyboard Shortcut box and then press the keyboard shortcut as you would when using it.
[6] Click Add to finish.
In this example I have remapped Paragarph to be Command + M instead of Minimise Window in Microsoft Word. Of course Command + M for minimise will still work in all other Applications.