I haven't found a good answer yet, but I'm still looking into it when I can.
(& yes, everyone -- to preface, I'm using a child theme like a good little strangeshe ;)
I applied the CSS changes outlined in ticket 18024 linked in this topic, above. That fixed the image sizing and positioning problem in IE when the featured image was the large 1000 x 288 size (but see new problem at end of this post). Prior to applying the changes from the ticket, my images (when they appeared at all) were extending beyond the left margin of the featured post 'box'.
Now that I've applied those CSS mods, the images are positioned correctly but still display inconsistently in IE. Occasionally, the first one or two slides will display as expected on initial page load, but after that, the 'only-on-mouseover' issue asserts itself. This is true with and without the auto-slide mod, which tells me that the problem is not in the js from Johan van der Wijk and archondigital in this topic: http://wordpress.org/support/topic/theme-twenty-eleven-auto-slide?replies=22#post-2356367.
I suspect the problem is in the original JS that came with Twenty Eleven. And I further suspect it has something to do with IE being touchy about the "visible" setting. But I'm a total newbie when it comes to JS and jQuery, so I can't be sure.
The IE8 problem can be viewed on my test site without the auto-slide, here: http://www.wikedtribe.com/
And on my regular site with the auto-slide, here: http://www.tadwilliams.com/
Both sites display properly in Firefox, Safari, Opera, and Chrome. I still only have XP, so don't have IE9 to test with.
Since the auto-slide part does not seem to make any real difference with this problem, I have not applied realepicurean's code to the header or js file (from same topic linked above) to select auto-slide by browser.
NOW, though, I have a new problem that I've not had time to chase down -- and this problem shows up in all browsers. After applying the CSS changes from ticket 18024, the large image positions were fixed, but the small images broke. You can see the problem on Featured Post 'slides' 3 and 5 on the tadwilliams.com site, and on slide 2 on the wikedtribe.com site.
The small images that go with those featured posts appear cropped -- they are actually 450x300 and 434x230 and can be seen in full if you View Image. Something in the CSS is pushing the images into the right margin, but the overflow is hidden so it looks like they're being cropped. Since it's happening in all browsers, and only began after applying the CSS changes from ticket 18024, I know it has to be a CSS problem. *sigh*
So, yeah. Still have IE8 issues with the Featured Post area (whether or not it auto-slides), and now also have new issue in all browsers with the smaller featured post images.
Haven't given up yet, though. I don't want to give up on twenty-eleven, just want it to work properly.
Thank you for reading this far and for any help you can give.