Opera 10 seems to have issue with borders in this test case. I have yet to narrow down the exact conditions, but this is a distilled example.

If the background of the li's is changed from "#DDD" to a color keyword, the border appear.

If all borders for the dd are applied together, the border will appear.

If no background is applied to the parent li - the first three dd's will lack a border - but all others will have their borders applied appropriately.