I posted earlier today a patch for the 2.9 firmware ipdf viewer that throttles CPU between 400 (when active), 200 (scribbles) and 100 mhz (idle). Still haven't gotten any good numbers as to how much battery time I get, but after 3 hours of combined reading and just being on, I've just lost one tiny line on the battery.
The post over at irex forums is at
here
Both a patch and a pre-compiled ipdf binary is available.
/R