Not a bad idea, but how fast is the service?
I'd bet that not many couples plan sex hours in advance -- the same reason that Viagra can be problematic, it takes a few hours.
I suppose, though, that if the delay is less than an hour or so, people in the mood could engage in long-running foreplay while waiting for the condom man to make the delivery. Or people could try to predict....but wouldn't it be humiliating if you thought you were going to score and secretly called in an order for condoms, and then your partner answered the door? It could get someone in the mood, I guess, but it sounds rather messy.
Though for people who aren't needing emergency service, this is still a very good idea. There are those who are ashamed to buy condoms, for some reason.