As you can see here on four-thirds.org many companies joined the Four-Thirds coalition a long time ago. The current question is: will the same companies produce a MicroFourThirds camera?
Sigma:
We received the rumor from two different sources. Both do confirm that Sigma will not make any MicroFourThirds camera. This doesn’t mean that they will not make MicroFourThirds lenses. But the interesting thing is that apparentely Sigma developed a MicroFourThirds camera! Sadly the final decision has been to not produce it for the mass market. We already told you that Leica and Fuji were also playing with the idea of joining MicroFourThirds but they have choosen not to do so. According to one of our sources Fuji developed a FourThirds sensor.
It is very difficoult to get reliable rumors from other companies but if the (many) rumors we received during the latest months are correct no other company beside Olympus and Panasonic will make a MicroFourThirds camera. No Kodak (read our Kodak rumor), no Leica (Leica rumor), no Fuji (Fuji rumor) no Sigma.
Not good news.
P.S: Please contact me at 43rumors@gmail.com if you can confirm/deny the rumor.