ALSA 1.0.17a contains a few updates in alsa-lib Core and the PCM API. There isn't anything major in this release, but the change-log can be found on the ALSA Wiki.
ALSA 1.0.18-rc1 on the other hand has a plethora of changes. One of the drivers that interests us the most, the C-Media CMI8788 Oxygen driver used by such sound cards as the Razer Barracuda AC-1 and other high-end products, has picked up support for the ASUS Xonar D1 sound card. The Xonar D1 was only released a few weeks back as a high-end 7.1 PCI sound card.
This 1.0.18 release candidate also has dynamic audio power management improvements, many new codecs as a result of Google's Summer of Code project, and many other changes. The detailed change-log can be read here.