Queen of Blades

I had always known that Glynnis Talken Campbell had lost her job as Nancy Sarah Kerrigan's voice actor in the years between StarCraft and StarCraft 2.
Fans who attended BlizzCon last year or watched the Kerrigan reveal trailer on YouTube were overjoyed when Campbell's voice purred from the shadows towards the end of the clip, assuming that it signaled a return of the character's original voice. Sadly, this isn't so. According to Campbell, the voice work done in that trailer was considered an audition for the role, and Blizzard has decided to go in another direction.

Glynnis has now confirmed with StarCraft Legacy that Blizzard has red-lighted her reappearance as Sarah Kerrigan.
Well, it just figures, doesn't it? Blizzard just gave me my Dear John phone call. Basically, it was "We can still be friends—we just don't want to see you anymore." Ha ha! Ah well...that's showbiz. Anyway, thank you for a wonderful and touching article—it's a great going away present, and I'll cherish it.
Along with this sad news, it also seems that the original voice of Jim Raynor, Robert Clotworthy, will not be reprising his role in the sequel either, though Raynor's increased age and newly acquired drinking problem might have played a part in the new casting.
I had heard the rumour before that Campbell's increased age and cigarettes had been the reason for the switch. The Kotaku article claims it apparently applied to Clotworthy as well, however Clotworthy is the Raynor voice actor in both games.

In the GameFAQs article I linked to above user DSMobster quips:
They used her voice in an early trailer and it sounded fine.
Then they told her to **** off and got a new VA because they wanted to make sure that the voice was as stereotypical as her dialogue
GraniteOctopus follows up with:
simply put they put in tricia helfer because shes famous and stupid people want to do things to her.

For those wondering what things people would want to do with Tricia Helfer...