I believe @shrike is talking about setting a fixed price in fiat before the offer is accepted. Is it not the case that when creating a buy offer, only the buyer's fees are set aside, and that none of the bitcoin that will be bought is escrowed yet?
I too wished for this when I was buying, as I wanted to spend a certain amount of fiat, up to a maximum, without being too concerned about just how much BTC I would get. I could make a dynamic offer and specify a fiat price, but that just got converted to a fixed BTC amount, so I might get stuck spending more than I wanted if the market price went up too far. If, instead, there were a mode where an offer were made in fiat, then the amount of BTC would become fixed when the seller accepts the offer.
Of course, I realize this might entail modifying the protocol and such a bit, creating a new offer/trade type or whatever. Alternatively, I suppose it could be acceptable to have offers that get canceled automatically if either the market price or fiat amount to be paid reaches some threshold.