This is useful for all sorts of stuff including displaying inline a row of images and captions in the same block , or as a container for a dropdown menu. But older browsers, notably Firefox 2 and Internet Explorer 6 and 7, do not support it. See the Mozilla Webdev blog for an explanation and fix.
In summary you can add "zoom:1; *display:inline" to trigger haslayout for IE and exploit the "*" CSS hack to make the thing inline.
But if you look at Quirksmode's always helpful compatibility page, you'll see that " "
So if you are not worried about Firefox 2 (anyone still using that? why?) you can simply make the inline-block container a span instead of a div and not worry about any hacks for Internet Explorer.
The trouble with hacks is you can never be sure they are proof against future browser releases.