Common usage defines a "liar" as someone who occasionally produces answers that differ from the known truth. This presents no problem at all: the poet, while lying occasionally, this time spoke the truth.
However, most formulations of logic define a "liar" as an entity that always produces the negation of the true answer, that is, someone who lies always. Thus, the poet's statement cannot be true: if it were, then he himself would be a liar who just spoke the truth, but liars don't do that. However, no contradiction arises if the poet's statement is taken to be false: the negation of "All Cretans are liars" is "Some Cretans aren't liars" (see [DeMorgan's laws]?), in other words: some Cretans sometimes speak the truth. This does not contradict the fact that our Cretan poet just lied.
Therefore, the statement "All Cretans are liars", if uttered by a Cretan, is false, but not paradoxical.
Even the statement "I am a liar" is not paradoxical; depending on the definition of "liar", it may be true or false.
See also: