Код, который выдаёт Youtube для встраивания <a href="http://www.youtube.com/watch?v=z_ux4T9k6wU">видеоролика</a>. Фулскрин работает.
<div>
<object width="560" height="340"><param name="movie" value="http://www.youtube.com/v/z_ux4T9k6wU&amp;hl=ru_RU&amp;fs=1?rel=0"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/z_ux4T9k6wU&amp;hl=ru_RU&amp;fs=1?rel=0" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="560" height="340"></embed></object>
</div>

Код с <a href="http://www.3dnews.ru/news/Apparatnie-trebovaniya-Android-30/">3dnews.ru</a>. Фулскрин не работает.
<div>
<object classid="clsid:d27cdb6e-ae6d-11cf-96b8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,40,0" width="560" height="340">
<param name="allowFullScreen" value="true">

<param name="allowscriptaccess" value="always">
<param name="src" value="http://www.youtube.com/v/z_ux4T9k6wU&amp;hl=en_GB&amp;fs=1">
<param name="allowfullscreen" value="true"><embed src="http://www.youtube.com/v/z_ux4T9k6wU&amp;hl=en_GB&amp;fs=1" type="application/x-shockwave-flash" width="560" height="340"></embed>

</object>
</div>

Код с 3dnews.ru + allowfullscreen="true". Фулскрин работает.
<div>
<object classid="clsid:d27cdb6e-ae6d-11cf-96b8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,40,0" width="560" height="340">
<param name="allowFullScreen" value="true">

<param name="allowscriptaccess" value="always">
<param name="src" value="http://www.youtube.com/v/z_ux4T9k6wU&amp;hl=en_GB&amp;fs=1">
<param name="allowfullscreen" value="true"><embed src="http://www.youtube.com/v/z_ux4T9k6wU&amp;hl=en_GB&amp;fs=1" type="application/x-shockwave-flash" width="560" height="340" allowfullscreen="true"></embed>
</object>
</div>