I'm using power49 withd vdd1 and vdd2 enabled at 900 Mhz for some days now.
Works well expect of unlzma'ing big archives. To be more precise, dd .. or tar -cfz (-xfz) of my 3 Gig debian partition works without issues.
Only lzma gives reboots even with nice -n 19.
I have tested it with this archive: http://qole.org/files/maemo-sdk-v1_2.img.ext2.lzma
Switching SR VDD off for lzma tasks like this is quickly done.

And the battery-savings of VDD2 are really good. A big difference, compared to only-vdd1-mode and directly noticeable.