l337:
1 ('one') or l ('el') = L
3 = E
7 = T
So you get 'LEET', which is the shortcut for 'ELITE'. The first 'E' is barely audible, hence the abbreviation.
h4x0r
h = H
4 = A
x = I think classical Dutch/German 'ch', which is pronounced as KH (think 'Christ')
0 = O ('oh')
r = R
leading to 'HAKHOR', which sounds close enough to 'hacker' if you swallow and twist the final syllable a bit. For a really good puzzle on what '1337' looks like, read
this and the following installments of GPF. Which is a really cool online comic, although I think Sluggy Freelance and Kevin & Kell are better. But now I'm digressing.