One very frustrating experience that I encountered after upgrading to OS X Lion (10.7) was that Safari 5.1, as included in Lion, would constantly pop up basic-authentication dialog boxes on our CMS sites, which are based on the Zope Toolkit (kind of between Zope 3.4 and Bluebream). Just about every page in our admin UI would do this. To get around this, I switched to Firefox for interacting with our admin screens, but still used Safari as my primary browser (better OS X citizen, takes advantage of Lion features which I enjoy, bookmark syncing, etc). This lead to problems as Firefox could get pushed way back in the usage stack and would be paged out, and it did NOT like to wake up after long periods of inactivity.
A couple of days ago I decided to take another look at the WebKit project as I was certain that I was not the only person having this issue. And I found a bug that had been recently closed relating to this issue. It's WebKit bug 66354 and it's resolution is in the WebKit nightly builds since at least September 2, 2011.
Apparently this only affects OS X Lion as it has to do with low level CFNetwork changes in Lion. The bug occurs not with basic auth, exactly, as I was able to use other systems behind basic auth. The bug occurs when there are redirects with Basic Auth, which our CMS uses a fair bit in its admins for basic navigation links.
As of OS X 10.7.1 and its Safari (Version 5.1 (7534.48.3)) this is broken. If you use OS X Lion and Safari and encounter HTTP Basic Auth problems, I'd recommend switching to the nightly builds.