Low light issue is fixed... Firmware upgrade... http://www.pentax.jp/english/support/digital/k-5_s.html (Thanks Andrew)

PENTAX wishes to announce the release of Firmware Update Software Version 1.03 for K-5.
Firmware in this document is the software that runs inside digital cameras.
Changes to V1.03

Improved the accuracy of auto focus in low-intensity light condition.
Improved stability for general performance.

*Contents of [Version 1.02] will be also updated.

Changes to V1.02

Compatible with SDXC memory card.

* Transfer rate is not compatible with UHS speed class.
All ISO sensitivity setting can be used on Bulb mode.
Improved functional sequence for Contrast AF and improved total speed until in focus.

Changes to V1.01

Increases the number of continuous shot at RAW format and High sensitivity shooting.

* Ex. Setting condition: RAW/ISO100, Increases the number of continuous shot from maximum approx. 8 frames to approx. 20 frames.
Improved stability for general performance such as exposure range on HyP mode