If you hold down a letter on an iPhone or iPad virtual keyboard, a list pops of of accents to put above the letter.Now in Lion, it's the same. Hold down any letter while you're typing, and a list pops up of accent options. Type the corresponding number to select which accent you want, or just click it.
It doesn't work in every app just yet (like Chrome), but it already works in many apps (like Adium, and apps that ship with Lion).