I am in the exact same situation as you.

I have pre-ordered Sins of a Solar Empire Rebellion on 07/04/12 from steam. Downloaded, installed the beta game through steam and created a Stardock account on the stardock.com website.

When I click to play the game a minimized window comes up titled Stardock Account Registration, enter my stardock account email and password then click register. Sometimes the window disappears before I can enter my details but when I enter them quick enough by pasting in my email first then typing my password in very quickly it shows this message "sins of a solar empire rebellion registration was successful"

So when I click to play the game again it shows the same minimized window titled Stardock Account Registration asking me to put in my details to register again when I already have ?

I submitted a ticket to support@stardock.com and someone dealing with my problem with no success yet.

If you do manage to solve your problem please message me back.

I found out that my anti-virus software (avast) which I initially thought it was my firewall that was affecting the Stardock Account Registration window disappearance. So I deactivated anti-virus for the registration process and now this message shows in a new window on top of the registration:

Stardock Account Registration

[Symbol = black explanation with a yellow triangle background]  "Maximum activation reached"

So then I press ok and the click close or quit then another message shows:

[Symbol = white cross with a red circle background]  "An error has occurred while activating the product"

Which I know means I tried to register the game too many times but still don't make a difference.

By the way did you get a serial number with the game because I never?

Crap, that must be it for me then as well- Avast! I mean.  I just turned off all the realtime protection shields and tried to load it up and it worked this time.  I guess I didn't try to register as many time as you did.

Also, I believe you don't get a serial key until the game officially launches.  When it does you'll be able to right click the game in steam library and select view cd key to see it.