I have everyone prepay me for Rails development. I think the reason it's not an issue is that they've worked with bad freelancers, and people that drop the ball, flake out, stop answering emails.

They don't want that so bad that they will prepay me because they believe I'm not going to do that to them, they really want me to do their project, and so prepayment is a non-issue.

Edit: I haven't asked for $30,000 in advance before; more like 20 hour blocks of time, or thirds of projects paid in advance