Okay i have a foot in both camps here. I have free use of a neighbours doppler which he kindly operates for me, i only use it really in the winter when its difficult to see seasons on gilts/sows so you're not sure if they've taken more so if you don't have your own boar to check them out for you. Technology is only a good thing if it works for you and your situation and its is each keepers perogative to do whats best for their stock, some people are comforted by the assurance technology can bring, i'm sure Microsoft will design a pregnancy detector phone app before long

. However nature as Robert says works in miraculous ways and pigs have been having piglets for thousands of years without any human or technicalogical intevention of any kind and will probably go on doing so.
Like Robert & Lill & BB i mark the service dates on the calendar and then use my trusty GOS year plan to claculate the likely due date, they usually farrow four days over it so i'm never far out.
Even with a scanner you should still be marking your dates on the calendar, its not going to forecast when they're due and this is part of pig husbandary, keeping records and planning ahead.
i hope this is a balanced view and that all participants can agree to differ, everyone is entitiled to their opinions and long may that continue.
Mandy
ps Tiz whats an abacus???