Flip to silence is not a new feature. Nokia first implemented it in its Symbian devices a number of years ago, and added the feature to its Windows Phone 7.5 devices last summer, as part of a minor software update. However, the feature did not make the jump to the Windows Phone 8 Lumia line up, presumably due to time and development constraints.
Incidentally, it's not a question of Windows Phone 8 GDR2 introducing the necessary enablers for the feature. HTC's equivalent "attentive phone" feature was available on both the 8X and 8S from day one. Indeed, HTC's implementation is superior to that of Nokia, as it offers "quiet ring on pickup" (lowered ringer volume on picking up the phone) and "loud ring in pocket or purse" features, in addition to its own "flip to silence" implementation.
The flip to silence enhancement is part of the Lumia Amber software update. This software update ships with the Nokia Lumia 925, but will also be coming to existing Nokia Lumia Windows Phone 8 devices as part of an over the air software update.
The update will be delivered on top of (i.e. at the same time as) the Windows Phone 8 GDR2 update. Nokia have said that the Lumia Amber update will start rolling out for existing devices (Lumia 520, 521, 620, 720, 810, 820, 822, 920) in the "summer months", which would suggest a July or August initial release date.
Do note that operator approval will be required for some variants, so, as with other software updates, it may take some time for the roll out to be completed.