A demo at this point wouldn't make sense.

Members of the human race are in general not rationnally thinking beings, even more so when it comes to the internet.
So when they see what is obvious to the beta testers, and that is a constantly evolving, slightly bugged, slightly unbalanced game, that's missing 2/3 of the races and several other things, they'd instantly, without thought, draw the red card and open the floodgates for little else then a whine fest.

"ZOMG dis game is soooo lame, lolz!"
"lolz, i played fur 5 min and then unsintall,ed, pos game lolz!"
"loool dis game has crap grafixs!"
"haha lolz rolfez lollerskates rolfcopter dis game suxxxx!!111one"
"Inset mouth breathing comment of choice here"

ect.

Sins is fairly solid however, and provides a decent amount of entertainment at this stage, but it's a long shot from the state it'll no doubt be in by Febuary 2008.
Also Ironclad and Stardock are smaller development houses, without mega corporations such as EA/Eidos/Vivendi ect to fund the development of their games.
The pre-order ensures that they get some additional funding during development, and no doubt also helps to ensure that they are even capable of releasing the product come 2008. Plus it gives an idea of how many are interested in the game and the idea of what it will be when it's finished.
The per-order also serves as an incentive to the people who pick it up, in the way that they get to help iron out any potential bugs/balance issues, and to a lesser degree push the direction of development in one direction or another through suggestions.

LOL

well said Novaburst and Vandenburg

Anyway, here's three of the most recent Previews/Hands on articles available.

Gamespy

IGN

Gamespot

And besides that, the Multianna has several short in-game captured clips from the various beta stages.

Multis SoaSE Fraps Corner

Some of the frapped videos also sneaked their way over to Gametrailers

Multianna comes around to many places!

I think what the OP meant is to have a beta similar to Supreme Commanders.

In supcoms , you didnt have to pre-order. You just had to get a key and join for free.

Those who did apply for the key to join were mostly beta-testers who were expecting bugs , broken gameplay and stuff needin fixes . However there were a few "10 year old kids" who did nothing productive for the beta.

Thx for the Links Nova, I was somewhat unsure about the game. All I wanted was a Gameplay clip and Yahoo didn't have any when I looked...   But now I am silenced...
The game looks awesome...  I justed wanted more info on the game, and I got it, thx