First all, the Clinton campaign has said it will not try to poach Obama’s pledged delegates.
Second of all: In the whirlwind of research I did trying to figure out how you’d go about poaching anyway, I came across an interesting coda to the true fact that “pledged” delegates can vote for whomever they want. They totally can. But candidates can also “remove delegates who they feel may be disloyal.” So…. the system works! Whee!