Ha, I had a similar thought not too long ago and was wondering about the pros and cons
Not sure if such a feature would be widely adopted since probably most merchants still have to buy the products / services they are offering in other currencies than BTC themselves. So if they accept bitcoins at a real-time exchange rate, they risk losing money if BTC drops in value after the purchase. Especially if they aren't early adopters and haven't already built up some significant BTC deposits to compensate for such a case.
So as I see it, there should at least be the possibility for the merchant to apply some (configurable?) "smoothing" to the exchange rate being used as well as some kind of add-on buffer to compensate for the currency risk. E.g. "current USD/BTC exchange rate plus 15%".
Also, you'd have to find a way not to be dependent on a single exchange like mt.gox for determining the current exchange rate, in case they temporarily go offline once again. Maybe build an average across multiple exchanges? I dunno.
Unless the exchange rate becomes much more stable, I guess this would probably mostly be for the bold or optimistic / idealistic merchants out there, who trust in BTCs mid- to long-term stability. And maybe the primary target group could be those who are offering services with relatively low expenditure in other currencies, such as artists or software developers.