Thank-you for your post!  I was having the exact Safari 3 problem you mentioned, with ScriptLoadFailedException's during partial updates.

I used just this snippet in a ScriptReferenced .js, and it fixed the problem for my setup:

Sys.Browser.WebKit = {}; //Safari 3 is considered WebKit
if( navigator.userAgent.indexOf( 'WebKit/' ) > -1 )
{
  Sys.Browser.agent = Sys.Browser.WebKit;
  Sys.Browser.version = parseFloat( navigator.userAgent.match(/WebKit\/(\d+(\.\d+)?)/)[1]);
  Sys.Browser.name = 'WebKit';
}

Is there a "more correct" way of removing the Safari hacks, instead of messing with the Sys.Browser object?