If I were about to take the momentous decision to go digital, in other words, get published by Kindle Direct Publishing and I had written an acknowledgement piece, would I need to contact the people mention in said acknowledgement in order to get their permission?
i am interested in working with you.
I'd say it depends on the type of acknowledgement.
If you're thanking personal contacts for helpful input or paid-for services, I bet they'd be only too delighted to get a mention.
You only need be concerned about Kindle's requirement for disclosure if there is any copyright risk involved. Cover art, for example, which may be used under license rather than you holding copyright. Illustrations. And excerpts from other peoples' works.
Of course it's always polite to check first with anyone you don't know that well :)