I noticed a difference, albeit a very small one, but was in library at the time...so in "normal" environments i guess its probably not noticeable.
Probably more like battery seconds? I guess lower rpm=lower voltage? So maybe some power being saved there, but I noticed my temps start to jump up faster now when doing anything of consequence, probably due to the fan speed being set lower initially, so the fan revs up earlier than if the min was 2500rpm...any power saving is probably offset by this. Maybe its even worse...though i guess idle/standby times would benefit if it stays at 1800 for any extended period of time?
Lots of assumptions there :S