Yes you've read it correctly BETA. We're talking about CS:S ofc! VALVE has ported CS:S to the newer so called Orange Box engine. The Beta is a closed one and everyone who was quick enough to sign up for it as soon as it was released got a slot for it.
What is the Orange Box (OBE) engine?
Games like TF2 which was the first release of the new engine use it. You can also see it in L4D1&2.
Have there been games already ported to it?
Yes and they've been a success since that happened. The already famous Day of Defeat: Source was ported to the OBE when TF2 was released.
Will this effect gameplay?
Tremendously. The HitBox bug seemed to be fixed with the new engine as it was written for Tick 66+ while the now old Source Engine in itself only supports a tickrate of 33. But it is still not comparable with the oldest of all VALVE games the nowadays called "GoldSource" which was the Quake engine and used for good ol' Counter-Strike which needed accuracy of highest degree.
When will they release the new Engine Update?
Call it
VALVE time.. But probably like the Steam 2010 update, in a few weeks after major bugs are taken out.
Until now you can report any Bugs you have found in the Beta here and we're going to compile them and send them over to Steam so they do not get flooded and as usual posts over in the forums @ steam get filled pretty quickly.
Where can i read more about the update and the Beta?
Right from the devils' mouth @ steampowered.com 
