You mean a gift certificate?
It's not so obvious how to do this. As you mention, the UI takes you directly to the option of charging it to your default credit card. I guess it's something to ask support about.
BTW, I recommend posting this on the General Troubleshooting forum, as it doesn't really qualify as as post that is "for potential DreamHost customers to ask current customers about their likes, dislikes, and general impressions with the service".
Use the [color=#CC0000]3DOM50[/color] promo code for 3 extra lifetime domains and $50 off
More Dreamhost coupons here!