The Boston Globe seems to know who's playing the Powers girl this time.
”We're told
Gisele Bundchen, who had a bit part in The Devil Wears Prada, is being considered for the female lead in the new Austin Powers film, following in the footsteps of
Elizabeth Hurley, Heather Graham, and
Beyoncé Knowles. Oh, behave!
Word is the 27-year-old Brazilian beauty already has been given the script, and her agent will meet soon with star
Mike Myers and director Jay Roach”.

Mike Myers wanted this sequel to come out in 2008, but seeing how he's got this summer covered with The Love Guru, it's more likely that Austin Powers 4 will be his 2009 hit.
As for Bundchen, the world's richest supermodel is extremely busy with photo shoots, fashion shows, and clothing lines, and she must find time to make a movie. She's obviously still up in the air.