Can data on a notebook's hard drive be affected or corrupted by a cell phone or iPhone's speakerphone or microphone which has a magnet?

"I suppose there's a theoretical risk, but I don't think the magnets in small speakers are strong enough to affect hard drives - unless there was something wrong with hard drive. You might just have a problem if you put a top of the range hi-fi speaker (something from B&W or B&O springs to mind) right next to the hard drive, but you'd have to take the speaker out of the cabinet first.

Magnetic fields obey the inverse square law in that doubling the distance from the speaker will reduce the field strength by a quarter, so the magnetic field doesn't extend very far at all.

I would also imagine that notebook and hard drive manufacturers have thought of this and ensure that there's no problem here."

There are (relatively) huge magnetic fields surrounding a HDD, you can run a standard size magnet right over one and it'll do nothing at all (much to my dismay last time I tried!), something from a phone is perfectly safe.