If Sega were re-entering the hardware market, they wouldn't have put out a Sonic game so recently. They would have saved it as a console exclusive. I'm also pretty sure that with the current global economic situation, and the slowdown of hardware advancement throughout the industry, console developers are going to hold off on new consoles for a while yet. We're going to get more products like the DSi, with new features and added capabilities.
Also, hardware isn't profitable anymore. Everyone but Nintendo is selling below cost and hoping to make money through software. They won't be willing to start new development cycles and disrupt software releases to cater to new systems until they run out of ideas for the current generation. Trying to compete with entrenched systems like these with vast game libraries would be suicide in good times. Current consoles won't be leaving anytime soon, and new consoles wouldn't make money anyway.
Bethesda ended up moving their Xbox exclusives to PS3 eventually. MGS4 might even get a port, if it's anything like the other MGS titles. From the quality of the last five Sonic games I know the in-house Sega studios can't be trusted, and third party games would come out for the systems I already have. Why would I ever want to buy a Dreamcast 2?
All things considered, 2009 would be the worst time I could ever imagine to release a new console.
