I have the exact same issue and mentioned it another thread here: http://forums.adobe.com/message/4129130#4129130

For me it is a huge loss. It helps for a number of reasons. Firstly, I like to snap in and out of Lights Out mode during editing to visualise the finished image without distraction. I also like to crop in lights out mode. While attempting to crop it is now much harder to locate the edge of the image under some circumstances. Additionally, it is very hard to discern where the image ends and the background lights out effect begins in images that have dark edges. It's 100% clear to me that this should be an option rather than a feature that is stripped out entirely.

As far as bugs go, I use lights out mode at least once per image during an edit. Often many times. Having used LR since release 1 I have never experienced an artifact left over by the lights out border. I'd very much like it back!

I find the white border very distracting... It's nice to have it gone.

Maybe a preference switch could be the better solution: activate it for the users who want and not for the others.

Yes Please Can we have this back I also use in the same manner as above

I have been viewing several images in both LR3 & LR4 in the Black, White & the Gray Lights out backgrounds & have the following suggestion.  Make a White outline, as in LR3, toggable. But also make a Black outline, toggable, available for the White background. You can see this effect in the Print module. If available there why not in Lights out? If you feel that using the stroke in the print module will make your image better, why not make it available in Lights Out for preview.

I've just upgraded also and on the flipside am gutted to find the border gone for good. I do most of my cropping in lights out mode and finding the edge of the frame can be tricky when the edge of the frame is a similar dark colour like the lights out background. Very annoying from my POV. Please implement it as a toggleable choice in Preferences>Interface!