Well, if this is true, I imagine we'll find out for sure at next week's GDC. That's when they're likely to announce it officially - if true.
Ooooh, I really, really, really, really hope it's true. And I really really really hope it's not as dire as Societies was. SimCity is one of my favourite games, ever, so a proper sequel would be really lovely.
SimCity 4 was actually one of the best games in the series, I thought. It was more realistic, and a lot harder (maybe a tad too hard at times) - I generally thought it was very good. Especially with the Rush Hour expansion. SimCity 2000 is still the best though.