Robert Meyer Burnett, who produced and directed behind-the-scenes material for Superman Returns, says that March, 2008 is the planned start of principal photography for the sequel. A Q&A on BlueTights.net reads:
Angeloz asks if I’m coming back to work on the sequel. The answer is…I’m ALREADY working on the sequel. Bryan, Brandon and the whole merry bunch will be coming back…and filming will probably begin March, 2008, as was always planned from the beginning. The WWII film Bryan plans to shoot over the summer will have NO EFFECT whatsoever on the Superman sequel schedule.
Doesn’t sound like there will be any kind of delay, as was suggested by Variety.