Arvind Bhatia, analyst at Stern Agee, is speculating that Nintendo will unveil their next console (usually referred to as "Wii 2") at E3 this year. He doesn't expect Sony or Microsoft to bring anything new out in the next two or three years, but that Nintendo needs to bring out some updated hardware.
"We would not be surprised if Nintendo unveiled the specifications of its next console at E3 in June this year, followed by product introduction early next year," the analyst told Gamasutra. As usual with analysts, this is in no way an official announcement of any kind.