I expect the right border to be drawn on all divs, but when you resize the window smaller so the inline divs will wrap to the next line, the right border is not rendered on the right most div. This happens in v3 of safari as well as the nightly webkit build from 7/24/2007.
Created attachment 15692 [details] source file here's the source file in case you need it.
Confirmed with a local debug build of WebKit r24637 with Safari 3 Public Beta 3.0.2 (522.12) on Mac OS X 10.4.10 (8R218). Sadly, this is not a regression as Safari 2.0.4 (419.3) with original WebKit on 10.4.10 also behaves the same way.
*** This bug has been marked as a duplicate of 8392 ***