I am unable to reproduce such a problem in IE8. Full test case? Are you sure there’s not a layering problem causing some other transparent element to cover the clickable area?
Does setting background-image make a difference? What about to a transparent GIF?
ETA: Curious! It’s actually an IE7 bug. For me, your example exhibits the described behaviour in IE7, but in IE8 it’s only when in EmulateIE7 mode; in IE8 native rendering it’s fixed. You’ll generally want to make sure you don’t fall back to IE7 rendering by using a suitable X-UA-Compatible header/meta; however, yes, setting the background-image to a transparent GIF fixed the problem for me. Tsk, we still need the blank GIF even in this day and age, huh?