Using the same browser (Safari v5.1.4) on another Stack Exchange site, say Server Fault, doesn't have the same problem.
Since Safari v5.1.4 was just released, it's very possible it could be a problem with the browser. But given the fact that two sites from Stack Exchange are showing different behaviour, I'd suspect the website at this point.
My browser is vanilla (no extensions/plug-ins).
Can others confirm this?!
When viewing the question NSregularExpression and password I get the following warning (but I don't believe it's related):
pixel;r=627855533;a=p-c1rF4kxgLUzNc;fpan=0;fpa=P0-1610253758-1313464351283;ns=0;ce=1;je=1;sr=1680x1050x24;enc=n;dst=1;et=1331643039578;tzo=240;ref=http%3A%2F%2Fstackoverflow.com%2F;url=http%3A%2F%2Fstackoverflow.com%2Fquestions%2F9683656%2Fnsregularexpression-and-password;ogl=:-1 Resource interpreted as Image but transferred with MIME type text/plain.