Some folks I know have had problems with their iPhones locking up, usually (but not always) when trying to multi-task. While I’m not an iPhone guru (and won’t be for a while — I have a Nokia N75 on contract) I did find some tips on the Apple site while trying to help. Since it is possible this could happen when you’re not near a computer, it might not hurt to read the basics.

Phone not charged: If you see a screen that says “Charging. Please wait…”, keep iPhone connected so it continues charging.

Try restarting iPhone. To turn it off, press and hold the Sleep/Wake button until the red slider appears. Slide your finger across the slider to turn off iPhone.

To turn iPhone on, press and hold the Sleep/Wake button until the Apple logo appears.

Does iPhone appear frozen or stuck? Try pressing and holding the Home button for 6 seconds to close a frozen application. Then try restarting iPhone again.

To reset iPhone, press and hold the Sleep/Wake button and the Home button at the same time until you see the Apple logo.

If iPhone is not working as expected, it may be due to some of the content synced to it. Try selectively removing certain content that may be causing unexpected behavior such as contacts, calendars, songs, photos, videos, or podcasts.

To remove content, connect iPhone via USB and use iTunes to change the sync settings. Click on iPhone in the Source list, and select each tab to change what content is synced then click apply.

Tap Settings > General > Reset > Reset All Settings

[Note: This will reset all settings. No data or media will be deleted.]

Any further solutions could involve losing data, and you won’t hear them from me. You can go to the source.

Other troubleshooting topics.

iPhone support page.