[OJFord]

Oh interesting, I have come across that, but only one person (both uses it like that and misinterpreted it when we worked with people that ack'd, which of course I saw coming & could explain since I was accustomed to his use) - didn't realise that was common too. (I would use agh/argh/ugh for that personally, depending on precise feeling(!))

[fwip]

Cathy (from newspaper funny pages) would frequently use "ACK!" as an exclamation of distress/frustration.

[EvanAnderson]

This must be why my wife asks if I'm okay (or if I saw a spider, etc) when I reply to her text messages with "ACK".