In fact, both the A318 and the B737-600 are bad replacements for DC-9s. They are designed for longer range flights, while the DC-9 and 717 are rugged, 12 flights per day workhorses.
Aloha already told us that the NG 737 is NOT capable of doing what the 732s were... namely, interisland ops...
So that leaves us with, pretty much, the 717 and the RJs.
Remember, Northwest kinda surprised us with their DC-10 replacements. Who would have predicted replacing a DC-10 with a 757? An A330, sure, but a 757? OOoooooooookay.
I also REALLY hope they go for the 717, it's my favorite craft.