My WordPress also quit sending me email notification for comments around October 9th, even though I had not changed anything. I was running WP 1.5.1.3, and then upgraded to WP 1.5.2 on 10/17. But, the upgrade didn't fix it either.
I looked up the WP support forum, and found this thread at http://wordpress.org/support/topic/42963 , but I'm not sure if how to make that change, and whether if that change to the php email function would fix this.
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