People tend to use domains as a crutch. It's an easy way to get excited, and it's the easiest first step to securing your idea. It may feel real, and potentially point to a simple landing page, but it doesn't miraculously manifest your product. The problem being, a domain can't give you feedback. A domain can only point to things. And if your product isn't live yet, you're not pointing at anything.
There's no point stressing about acquiring or finding the right domain just yet. 37signals has been great about this approach. Even though their applications have common names like Campfire and Basecamp, their actual URLs are campfirenow.com and basecamphq.com. Don't let your domain become the limiting factor when building your idea. There's a ton more things worth focusing on.
Soon as someone registers their domain, their next step usually is to grab the Twitter handle. That's where the real benefit is. Not in the domain, but in the Twitter handle.
It's your first point of interaction with users and customers. It's the easiest way for you to communicate your product, idea or vision. It's your opportunity to grow an audience so when the product goes live, you have seeded users. Start building attention, focus and reach out to people of interest. Communication is far more valuable and effective than an empty domain that points no where and contains nothing. That's how you create attention and that's how you begin gathering early-adopters.
You want your product to be real, treat it as such. Communicate your product and vision so effectively, so clearly that people will think it's real. Convince me it's real, or at least manage to speak out about it in a way that I'm now genuinely curious to see it in action. Use your voice and use Twitter to your advantage. Forget the domain.