Adam is right - it often comes down to the person and if they WANT to be successful and put in the energy to "change". I think we can break bad habits but it is pretty difficult to change our entire approach to life in general.
Funny thing is, motivation is a fluid emotion, so even though I might have it today.... it could be gone tomorrow. Some employers spend a lot of time and energy "saving" someone who can't be saved while others toss people aside too quickly, when a little coaching might have fixed the issues.
Thoughts? How far should we go when employees are under performing? How many "chances" do they get?