Famke Janssen has always been an integral part of the original X-Men franchise as Jean Gray/Phoenix. Although her involvement in X-Men: Days of Future Past has yet to be confirmed, she herself says that she will be making a small appearance in the upcoming 'Wolverine' film with Hugh Jackman.
Astute fans first thought they spotted a glimpse of Janssen in the trailer for 'The Wolverine', and she herself confirms an appearance in the film. She says, "I couldn’t tell you [how much I’m in it]. It’s a flashback sequence, but that’s all I can say. You’ll just have to wait until July!”
She also talks about the honor of being part of a franchise like X-Men, saying, "I think that the X-Men series is a really successful one in constantly finding new and additional characters, or younger versions of the same characters to explore. I like what they’re doing. I think that they’ve always chosen really fine actors. With X-Men: First Class you had Michael Fassbender, Jennifer Lawrence and James McAvoy so it’s amazing the kind of actors they keep getting for these. It’s a classy series so I’m proud to be a part of it”.
If Janssen is part of Days of Future Past, we'll have to wait until Bryan Singer reveals some sort of tidbit to confirm the news. Until then, we'll have to wait until 'The Wolverine' releases on July 26th to get a glimpse of Famke Janssen returning as Jean Gray.