I also see (conky/top) that hildon-desktop+xorg ~ (10-14)%cpu, but it's only when the screen is on ... when I lock the device, ssh to N900 and run top the cpu usage by these two processes is gone, so I doubt that this can cause overnight battery drain ... I can also confirm that it's still present even if I remove all widgets ... didn't tried to remove cpumem applet yet ...

[edit] yes, cpumem applet did it in my case, after removing hildon-desktop ~ 0.x %CPU

[edit1] nope, it's back again ... :-/