the browser infrastructure is at the OS level? uh oh, didn't that get MSFT in trouble (it wasn't the only thing, but it was a signficant part of the charges)?
Re:Apple releases Safari 3.1
by thehigherlife on Tuesday March 18, 2008 @01:20PM
(#139971)
from wikipedia:
WebCore
WebCore is a layout, rendering, and DOM library for HTML and SVG developed by the WebKit project, originally developed by Apple as a fork of KHTML. It is licensed under the LGPL. The WebKit framework wraps WebCore and JavaScriptCore, providing an Objective-C application programming interface to the C++-based WebCore rendering engine and JavaScriptCore script engine, allowing it to easily be referenced by Cocoa-based applications; later versions also include a cross-platform C++ platform abstraction, and various ports provide additional APIs.
end quote
basically it has to update and restart so that all cocoa based apps can use the new version.
edit: maybe 'entire OS' was a little vague. basically they don't want to run into issues where the webkit gets updated and things that rely on webkit don't get all messed up in the process because they are running too.