The same behavior is also occurring with my WordPress installation (also on 1.5.1)
What I know is this.
1. Creating a new post and entering text using the Wysiwyg editor works until saving the post. Even using the "save as draft" option. The moment you save the post, all post content is gone and only the word "undefined" remains.
2. If you change the tab on the Wysiwyg editor from "visual" to "text" and then enter text and save, it works. This, however is without any formatting unless you do all the html formatting tags by hand.
This happens on both my live site and my dev site (which I use for testing) and I have disabled all plugins and changed to the "twenty twelve" default theme on my dev site and can still reproduce it.
I called GoDaddy (my hosting provider) and they didn't know anything about it yet (although I could be the first to report it).
I am going to set up a subdomain and a fresh install of WordPress with no additional plugins or themes and test to see if I can reproduce it.
If anyone knows more about the issue than what I have reported here, please update this thread.