Among the ACPI/PM changes coming up for Linux 3.9 include supporting a bit more of the ACPI 5 specification, a cpufreq driver for Marvell Kirkwood SoCs, support for new tables within the ACPICA iASL, performance enhancements to ACPI package objects, finally a cpufreq driver for the Calxeda Highbank, Lynxpoint LPSS clocks support, an imx6q cpufreq driver, support for the Intel Low Power Subsystem, and the new Intel P-State driver. There's also a lot of other random ACPI / power management changes scattered throughout.
The new hardware support / cpufreq drivers is great along with Intel's P-State driver. The Lynx Point chipset work is interesting and the Intel Low Power Subsystem (LPSS) is a new feature to this chipset coming for Intel's Haswell platform.
The current list of changes being queued up for the Linux 3.9 merge window can be found on the Linux kernel mailing list.
Other features being set for the Linux 3.9 kernel are covered in the aptly-named Features Coming In For The Linux 3.9 Kernel article.