eeer i may spoil some people here but you actually "talk" with the g-man at the end of HL1. He offers you to work for him and his organisation on the study of alien worlds etc. He seems to be working with (for?) the government, and I wonder if this has not been confirmed by valve itself.
He also is at the beginning of a mod which was distributed with CS retail in france, I didn't finish that one though. (forgot the name too...) But I remember of a looong intro taking place in the office of the g-man, where he keeps talking about alien things
For OpFor & BShift, it's quite logical to find him there as they're only HL1's story viewed from another side.
I didn't finish HL2 either, but it seems that we don't learn anything interesting in it anyway...
(the models likeness is a nice find
)