From all the stuff happening earlier in the week with the new portal update it appears that now the ending of the game has changed!
Thanks to MrSnuggleDuck (what a name!) for uploading this to youtube.
Of course the new bit doesn't happen until the very ending of the video where...
you get dragged off by, presumably a german robot. There's been a heck of a load of stuff going on with all this and there are a few people out there who have collated the data, so I'm not about to replicate this.
The best summary is on my favourite HalfLife related fansite: Halflife2.net where they've gone over the trail of breadcrumbs valve have left to be followed.
Looks like Valve have been very busy recently getting all this into place, plus their rumoured "Mac Version" of steam.
Gabe Newell, the Valve Supremo, will be talking at GDC2010 next week (11th March) so I'm sure we'll hear some interesting bits and pieces from that!