Well if you are going by the Wikipedia list the top ten PC publishers are:
1.EA
2. Activision Blizzard
3. Ubisoft
4. Take-Two
5. Bethesda
6. THQ
7. SquareEnix / Eidos
8. Microsoft
9 & 10 would be a tie between Sega, Capcom and Konami
Ubisoft are already on board so if you are talking top 3 Its EA, Activision or Take-Two. Out of those three the most likely is Take-Two who own Rockstar and 2K.

Thats pretty much what I said, its just that its sad to see such a big publisher want so much for their very old titles.

Yeah, ok. Fair enough, I agree with that. But the most important step is to get them onboard in the first place. Price policies change over time. :-)
Edit: Wait a moment... "early next year"? I got the impression the announcement would happen soon-ish?
Edit 2:
That would certainly make sense, and be great news as well. Take-Two owns the Sid Meier-stuff, for instance. Just think about that for a moment. :-)

Just so everyone knows, it's EA.

And you know this how? I am one of the people that say EA are trying to be Mr nice guy but on GOG? Hell naw.

Very reliable sources. Not 100% sure... but very likely it is them.

Well, the only other candidate is ActiBlizz who are about as likely to agree to GOG's terms as Rush Limbaugh is likely to say that communism isn't such a bad idea after all. We have the other top three publisher in the form of Ubisoft.

As I've already pointed out, that list has nothing to do with the size of the publisher. From the article:
"Note that this is not a ranking by revenue, but of the quality of experience of working with the publishers according to staff, and some video game development companies."

And as I pointed out IF you take the Wiki top 10. No need to correct when there is no need to correct.

No.
Sorry, I had to.

Yup. Every single one.

If the biggest game publisher comes on board, it's going to snowball as a lot of other fence sitters are going to mimic them.
Seriously, I can't understand why so many are that hesitant to put their older games on GOG. Most of them have UIs which are way too complex for casuals to use, or graphics way too outdated to ever have any kind of mainstream appeal again.

GOG is what you would call a niche site. It's not meant to have mainstream appeal, nor is it directly targeted at casual gamers.

I think the publisher argument goes something like this:
"GOG uses no DRM, therefore all games released there will be heavily pirated. As we all know, one pirated game equals one lost sale. This means that if we sell our games on GOG we will lose money, whereas if we don't sell them at all, we won't lose money. QED."
And no, none of that made any sense, or had any relation to reality, but I do believe that's how they think.

How would EA make money again with old Ultima DOS games or System Shock 1 and 2 without a site like GOG? At this point it's not going to happen, they're too antiquated.
Their games are just laying dormant and netting them NOTHING. Even if DRM-free GOG games are pirated, it makes them SOME money.
Plus there's such a strong sense of community here that many people practically buy everything on GOG because it's from GOG.