Some have observed the Release Notes in the SL Forums are behind the times—that's my doing, or rather, undoing. Apologies! I'm looking forward to 1.9.1 rolling out on Grid Agni so I can post the 1.9.0 in a single batch. Forums aren't very conductive to this type of thing.
And now for good news: Oz Spade, who's recently been commenting to my blog, does a FANTABULOUS job of updating Release Notes at the SL History Wiki. An easy top-down view, just find your version and read away. He's even got Preview 1.9.1 ones up, updated freshly after each iteration is released. Do yourself a favor, bookmark the page. I have.
Also, Oz informed me of Dyne Talamasca's contributions to the SL History Wiki. She even has a bio page, outlining her work. What a kickass team they've got. Read about Eggy Lippmann's proprosed plans for the future too!