I've written up a 2-page explanation and proof of Aumann's agreement theorem. Here is a direct link to the pdf via Dropbox. The document is also available on Scribd. (It can be viewed by anyone, but a Scribd login appears to be required to download, so I won't be using Scribd anymore.)
The proof in Aumann's original paper is already very short and accessible. (Wei Dai gave an exposition closely following Aumann's in this post.) My intention here was to make the proof even more accessible by putting it in elementary Bayesian terms, stripping out the talk of meets and joins in partition posets. (Just to be clear, the proof is just a reformulation of Aumann's and not in any way original.)
I will appreciate any suggestions for improvements.
Update: I've added an abstract and made one of the conditions in the formal description of "common knowledge" explicit in the informal description.
Update: Here is a direct link to the pdf via Dropbox (ht to Vladimir Nesov).
Update: In this comment, I explain why the definition of "common knowledge" in the write-up is the same as Aumann's.