This is a decent idea but there are a few caveats :

Wouldnt the receiver IP need to be online 24/7? If the server is down for some reason then I guess you need to implement some sort of fallback or it wont go through...

Man in the middle is awfully scary too, especially if you're relying on a server rsponse to send your coins. You havve to worry not only if you're the receiver but also the sender. There are a lot of attack vectors that you need to proof against, with SSL mentioned by satoshi above being just a small part of it..

People will be a lot more tempted to compromise servers. You cant fake a btc addresss or claim it unless you have the privkey, but you can definitely hack servers, the damage would be massive compared to just a single compromised address.. Same goes for clientside, you can use SSL to encrypt your requests, but what about before the request goes out? I know very little about networking and the dumbest idea I can come up with would be to just fuck with your hosts file and simply redirect all your requests from someexchange.com to mywebsite.hack..

If there is something like this in the works Im sure they're all beating the idea to death in order to avoid/minimize issues like these. Just see how cryptography has very little human factor involved and yet we still see people lose their keys. If you open up a door like this imo the benefits are minimal compared to how much more room there will be for human error.