Don't launch with a splash
If you follow tech blogs, you've seen a million of these - startup homepages that are nothing but an email address field and a vague sentence selling a vague idea. The hope is that you'll type in your email so you can be notified if and when the startup launches.
The big issue with the splash launch is we, as humans, simply don't have the luxury to care about everything we see. In a world of continuous partial attention, we jump from product to product, idea to idea, often several times per minute. If all you have is a splash screen and an input field, it is like you're bringing a stick to a sword fight. People will leave, and they will forget.
Your splash page is your pitch
If you're not ready to launch but still need some kind of web presence (and chances are you do), make it meaningful. Instead of passively trying to sell to people just enough for them to give you their email address (an increasingly complicated proposition, given how much stuff we get in our inbox already), sell the idea of the product from day one. If I can't have your product now, what can you do to make me want to wait?
Your most important pitch, is the pitch to your users. And that is what your splash page should be. It isn't about their email address - it's about you. It's about the problem you solve, and the wrong you are going to right with your app or service. Look at it from that perspective, build it with that in mind, and then ask me for my email address. Chances are I'll be happy to give you that, now that I know I have something exciting to wait for.