In this case it is nominalizing the verb 食べる - making it into something like "the act of eating". You can't directly make the verb 食べる the subject of the sentence - 食べるは is ungrammatical. So you first turn 食べる into a noun with the nominalizing の, then it can be the subject. So you get:

"(the act of) eating the same thing every day, (it's) not interesting."

P.S. 物 is a noun, you're right, but it is serving as the object of the verb 食べる and not as the subject of the sentence.

Last edited by Delina (2010 February 24, 8:37 pm)

If you haven't read it yet the taekim grammar guide has a nice explanation for how and why it works in more detail in it's basic grammar section.