Unless Firaxis has the manpower and have finalized the game already (see how unlikely that sounds?). You'll have to wait a month or so after release.
Imagine if they did release a demo before release, and you didn't like the game, they just lost you as a customer. It's all too risky.
I'd have to totally disagree. For any potential customer that doesn't like the demo and doesn't buy there's at least one potential customer on the fence who ends up buying it after trying the demo.
If you make a good product and you believe in it, there's no reason not to have a demo.
Except, that it takes manpower to make the demo version, it takes manpower/resources to distribute, and when you KNOW you have a hit on your hands, like civ5 of course will be, there's less reason to put out a demo, because you know you're going to sell copies.
I'd much rather they polish the game and take care of last minute details than worry about a demo.
That's why they put out a demo after, to pull in even more sales when the game is out, they've had some time to chill/take vacations, the inevitable first patch is out, and so on.
Companies like blizzard and firaxis, who put out killer game after killer game, have no reason to rush to create demos or trials. They will sell tons of copies regardless.
I just wish firaxis shared blizzard's stance on info. Blizzard KNOWS they kill. They know WoW slays universes (as does WC, SC, and Diablo series). They drop their NDAs months ahead of release because they KNOW they will sell and all the info that gets out just makes fans even more rabid for their products. It's like free advertising to them to drop their NDAs months before release. When you KNOW you deliver, there's no reason to be stingy with info.