SEO Tip #43: Will a “coming soon” page negatively impact my site?
Matt Cutts: No. I think a coming soon page can be pretty smart.
I think it’s good for users so that they don’t just end up on a black hole page that it doesn’t resolve or something like that. If you have some content that’s coming out I don’t think there is any harm from having a coming soon page and then as you get more content you can put that content out there.
Then when the full site is ready you can have the full site out there. So I wouldn’t worry about, ranking wise, that being a problem. I think it can be a good thing for users and it can be a good thing for search engines.