e commerce - Does highlighting the potential for discounts negatively impact on non-eligible customers?

I'm interested to know peoples opinions on this. Does clearly showing a voucher/coupon/discount code field within a quote or payment process negatively impact on the customer experience for people who don't qualify (e.g. don't have a coupon)?

Personally if I see a discount code field in a form the first thing I do is search for a code. I recently had this with Pixelmator and saved £7. Knowing I have a discount code prior to purchase makes me more inclined to purchase and positively impacts on my experience (e.g. Rosenfeld Media give UPA members a discount) but the reverse is true when I don't have a code and the potential is highlighted. It makes me feel that I'm some how missing out and that the company can afford to sell their product cheaper but I'm not seen as important enough to qualify.

Do you know of any research for/against this or had any experience of it through user testing etc?

Should discount code fields be made more prominent or should they be reduced to say a link that shows/hides the field?

Any feedback would be really appreciated.