I don't any next-gen console coming out anywhere before holiday of 2010, if not later.
Microsoft: The only reason the XBOX went 4 years in between cycles is because Microsoft wanted to get out a year early, and you see how that worked out (good and bad). However, I think it's quite possible that we won't see anything until 2011, depending on how Microsoft continues to do in the market. But most likely, Microsoft is prepping for a holiday 2010 release, and will likely beat the competition to market once again.
Sony: Things are definitely picking up for Sony. I can't really see them gunning to match Microsoft if they release in 2010. The PS3 still seems like a big work in process, and to have a short 4-year turnaround doesn't mesh with their planned 10-year product cycle. The PS4 will borrow heavily from the PS3 to reduce costs, and I can't see them innovating too much with it.
Nintendo: If things keep going the way they are, I don't see Nintendo in any hurry at all to come out with a successor to the Wii. When they do, I think it'll be 2011 at the earliest, possibly even later. And if the Wii is still popular in a few years (which it probably will be), it'll definitely be called the "Super Wii" or something like that 🙂, and it would be 100% compatible with the original Wii software and hardware. It would most likely use current-gen tech (considered to be old by the time 2011 or 2012 rolls around) to keep costs still lower than the competition. Nintendo has found their niche with the "casual" market with the Wii and DS, and I doubt they'll reliquish their hold on it anytime soon.
EDIT: This is just my personal speculation. I have no info or sources to make my opinions reputable 😛 😀.