I believe that Gedo Mazo is the body of the Juubi and the reason the tailed beasts are being stored in it is to return the Juubi's power to its body.
We have been told that the Juubi's body was sealed in the moon. http://read.mangashare.com/Naruto/ch...7/page015.html however we have no evidence to suggest that it is still there til this day.
When Nagato (Rikudo sage's reincarnation) summoned the Gedo Mazo (possibly from the moon) he was able to control it as only the Rikudo Sage's should be able to, however it was still so utterly powerful that his body was greatly injured by this feat.
Sometime after Madara came along and joined forces with Nagato and they began the plan to bring the Juubi back to life. Since the power of the Juubi's body was so great, they couldn't risk losing control of it and so had to place greater binding on it http://read.mangashare.com/Naruto/ch...4/page016.html
Around this time I believe Kishi also forshadowed the connection between Gedo and the Juubi by using this picture http://http://read.mangashare.com/Naruto/chapter-254/page018.html particularly the prominance of the Gedo Mazo holding up his 10 fingers. Notice also "as if gods who subjugate the lord of hell" Well the Juubi seems like a great candidate to be the lord of hell.
Here is our only picture of the Juubi http://read.mangashare.com/Naruto/ch...7/page014.html and here is a picture of Gedo http://read.mangashare.com/Naruto/ch...7/page008.html notice that Gedo looks like an incomplete or damaged Juubi
There is a reason that Gedo Mazo has a blindfold over his eyes and that is because the Juubi's eye is under it
Thanks for listening folks
edit: oh and here (hhttp://read.mangashare.com/Naruto/chapter-255/page007.html is one more possible foreshadowing. Notice that there are 9 demons going to collect the one tail from Gaara (9+1= juubi )