Using the character filter on FF.net and sorting by publish date, the earliest Daphne fic of substantial size with a substantial readership was Harry Potter and the Marriage Contracts. However, the Daphne in that story is no Ice Queen. I think it was more likely The Grass Is Always Greener: the author's note at the end of chapter 1 indicates that the Slytherin Ice Queen cliche was already around but that her identity hadn't yet settled on Daphne. That may well be the fic that started it. The author formerly known as munkeymaniac did a lot to popularise the idea as well, although his original Harry/Daphne stories (Harry's Secret Friend and Dragon's Revenge) were only recently reposted so I don't know their original dates.

Whew... that was a journey through time.