Entering accented characters while using *any* keyboard layout is easier than ever, similar to using iPhone, iPad or iPod Touch.
Previously, to enter letters like é, ñ, ö, you had to learn one of several hints: option-key combinations, use the International Keyboard, use the Keyboard Viewer or change the keyboard layout to the appropriate one and then touch-type the keys you want.
But, 10.7 goes the route of the iPhone/iPod Touch/iPad software.
If you want an accent over the letter 'e' just hold down the 'e' key as if you wanted to type 'eeeeeeeee.' You will see a short list of accent marks, just find the one you want and type the number underneath it.
[crarko adds: I tested this, and it works as described. See this hint for some other ramifications of this feature.]
Mac OS X Hints