Available now, OS X Lion for Mac for $29.99 (you can also get OS X Lion Server for a $49.99 add-on after purchasing Lion).
At the price it's offered for, it's a no brainer to do the upgrade. I haven't done it yet because I haven't done a proper backup of my machine (you do backup your Mac, right?) but will be buying the upgrade as soon as that's done.
Even better for those of you with more than one Mac - as long as they're all using the same Mac App Store account, you can install the same Lion upgrade on all the machines. One thing you'll need to do before you buy the upgrade is run Software Update as there's a Migration Assistant update that is needed to help transfer all your data to the new OS. (Click on the Apple in the top left corner, click Software Update and install the upgrades)
If you'd like to support what I'm doing on this blog and are going to be buying Lion anyway, I'd really appreciate if you clicked the link above or the Mac App Store button below to buy your copy of Lion. I'll get a few bucks that would otherwise go into Steve Jobs' pocket to help pay for the server widgets that run this site.