Sony has virtually zero chance of outselling MS in the US outside of the next 4 weeks or so. Once ODST hits, it will be MS back on top. The holidays will handily go to MS because of the arcade bundles that are sure to come. $299 is still way to expensive at the moment.
In 2010, MS will probably bury Sony (and possibly even Nintendo) with Natal.
OUS, Sony will be ahead for the rest of the year. However, when Natal launches it will be up for grabs, although I give MS a better chance of winning EU than Sony. Japan is a foregone conclusions so it isn't worth talking about.
This is where I stopped taking this post seriously.
Japapn is still a VERY important market btw, whether you want to admit it or not. For Microsoft to rule this industry, they must rule there as well.
Anyone that doesn't think that NATAL will radically shift the balance isn't paying attention or is being deliberately obtuse.
The reason Natal wont affect sales much is because Natal is coming out a while after Nintendo's Motion Cam (UbiCam) and the expected timeframe for Sony's motion controls. Nintendo has the casual market in lockdown, they won't be migrating over just on the sheer fact that they already own a Wii, which has Motion Plus and will have the motion camera in 2 months time.
Sony fans won't be switching over to the 360 because - they're Sony fans. They are very loyal to their brand.
In the end, Natal will provide motion controls for 360 userbase, but it won't do much to expand their marketshare.