Greetings:

I come from the "mainstream" business world so to speak, and I can tell you, asking for 50% up front is the LEAST that many good designers expect.

Many times when I've hired people to create sites/fliers/newsletters/ in the past, the designer would:

1. Would require 50% up front.
2. If it was a large order, require 100% paid up front into an escrow account.
3. Would deliver a proof to us to approve first that was in a format that couldn't be copied such as printed out, have the webpage captured in screenshots, or in pdf format. Then, require payment before releasing the html/flash/whatever files.
4. I even had one design firm (granted, it was a large New York office) require credit references if we wanted net 30 terms instead of payment in full upfront.

I never had a problem with any of it. If anything, I think it makes the designer look all that more professional. If someone's unwilling to accept a proof, or pay into escrow, you have to wonder if you'll ever see your money from them to begin with...