How do the clients agree on the target to hash for? Edit: They do not agree on a specific target hash. Any hash lower than or equal to the difficulty is sufficient. This means clients only have to agree on difficulty.
By "difficulty" I mean the number that determines the minimum acceptable hash. Higher difficulties mean lower hash minimums and thus, on average, more hash attempts needed to produce a valid block. (Related terms are target and nBits).

Every 2,016 blocks there is a difficulty change block. Unless the block is a difficulty change block, the difficulty must be precisely the same as the previous block or the block is invalid.

On a difficulty change block, the difficulty must be precisely what the difficulty algorithm requires or the block is invalid. The algorithm takes into account the amount of time it took to generate the previous group of blocks and computes a new difficulty such that the block generation time will be, on average, once every ten minutes if the network hashing power stays the same.

If you want to see this in the source code, check out the GetNextWorkRequired function in the main.cpp file.