For one thing, it depends on how long your runs are. Battery life is a big issue with the phone when you turn on location services, so it's not the greatest thing for cycling, but if you're running for less than an hour, the phone should be fine. Also, AFAIK no one is yet making the extra hardware for iPhone, so you wouldn't have HRM or footpod (or cadence on the bike).

Honestly, I haven't priced a 405 recently, but the new 310 has so many more bells and whistles, if it's not a lot more expensive I'd go that way.