I don't think there's any one answer that applies to all games here. I think it depends on the group you're playing with. No two are the same.
The impression I get from many of the MP threads that I've looked at is that some good natured OOC / metagaming / banter is not considered a sin by many players. In my opinion there's nothing wrong with a little trash talk. That said, I'm certain there are players who prefer to RP their nation to the hilt, and it definitely adds something to the game when people put that much effort into their game. But many of us have busy lives, so not everyone may be able to keep that up while handling mid or late game micromanagement in an MP game (one reason why some of the vets love Anonymous No Diplo games).
Aside from the tone of someone's forum posts whether they be in character or not, from a thematic perspective I think messages on the forums can be justified as "in-character" information about major events in the world percolating out to the various nations. Each turn represents a month of in-game time which sounds like enough time for word to get around about major battles. Or perhaps each pretender has a special magic conch shell* that connects him directly to each other pretender.
I think if you have a strong preference about something like this you'll probably want to start up / admin a game where you can set the expectations about it up front.
Here's a question for you: How does someone setting their nation AI fall on the metagaming front? There are people here who have railed against people going AI, and there are other people that see nothing wrong with cutting their losses when they're losing by going AI. How do you justify it thematically? Is it going to bother you when it happens?
--
*"Oh-ho, that's much better... yeah... huh... yes... Fine, I can hear you now, Dmitri... Clear and plain and coming through fine... I'm coming through fine, too, eh?... Good, then... well, then, as you say, we're both coming through fine..." -President Merkin Muffley
http://www.youtube.com/watch?v=6T2uBeiNXAo