Cannot enchant above cost 8 in MC 1.6

Enchantments appear to be limited in level/cost since updating to MC 1.6.

In MC 1.5.2 I was able to enchant items with enchantments costing up to the max of 30 on many occasions.

Since updating to MC 1.6.x no enchantments above level/cost 8 are available no matter how many times I place/remove the item in the GUI.

Reverting to 1.5.2 in the launcher solves the problem and I can enchant up to 30, but switching back to 1.6.x causes the problem to return.