They most likely use Ajax and HTML5 history.pushSate .
Please note that this only works in the latest browsers like Firefox 4 and Chrome 8 (IE 9?). Thus, you still need a backup solution, for example, to use the fragment identifier, since now many websites are managed by Ajax (a good example to see the difference on Facebook).
Felix Kling Jan 12 '11 at 0:20 2011-01-12 00:20
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