The iPhone can demand a password each time it wakes up, if you like.

You can’t use any other company’s SIM card in the iPhone – it’s not an “unlocked” GSM phone (at least, not officially; there are some unauthorized ways). Other recent AT&T cards work, but only after you first activate them. Insert the other card – it fits only one way, with the AT&T logo facing up – then connect the iPhone to your computer and let the iTunes software walk you through the process.

Geeks may enjoy knowing that the screen is 320 by 480 pixels.

On the iPhone 3G, you can use any standard headphones with the iPhone – a welcome bit of news for audiophiles who don’t think the included earbuds do their music justice. But on the original iPhone, the molding around the iPhone’s audio jack prevents most miniplugs from going all the way. You must be able to get your headphones to fit by trimming the plastic collar with a razor blade – or you can spend $10 for a headphone adapter (from Belkin.com, among others) to get around this problem.

Camouflaged behind the black glass above the earpiece, where you can’t see them except with a bright flashlight, are two sensors. First, there’s an ambient-light sensor that brightens the display when you’re in sunlight and dims it in darker places. You can also adjust the brightness manually. Second, there’s a proximity sensor. When something (like your head) is close to the sensor when you’re using the phone functions, it shuts off the screen illumination and touch sensitivity. Try it out with your hand. (It works only in the iPhone application.) You save power and avoid tapping buttons with your cheekbone.

With practice, you can learn to tell if the ringer is on while the iPhone is still in your pocket. That’s because when the ringer is on, the switch falls in a straight line with the volume keys. By swiping your thumb across these controls from front to back, you can feel whether teh silencer switch is lined up or tilted away.

There’s only one payoff for knowing what’s down here: The speakerphone isn’t very loud, because it’s aimed straight out of the iPhone’s edge, away from you. If you cup your hand around the bottom edge, you can redirect the sound toward your face, for an immediate boost in volume and quality.

You may have noticed one standard cellphone feature that’s not here: the battery compartment door. The battery isn’t user-replaceable. It’s rechargeable, of course – it charges whenever it’s in the white dock or connected via the USB cable – but after 300 or 400 charges, it will start to hold less juice. Eventually, you’ll have to pay Apple to install a new battery. (Apple says that the added bulk of a protective plastic battery compartment, a removable door and latch, and battery retaining springs would have meant a much smaller battery – or a much thicker iPhone).

Because the period is such a frequently used symbol, there’s a little shortcut that doesn’t require switching to the punctuation keyboard: at the end of a sentence, just tap the Space bar twice. You get a period, a space, and a capitalized letter at the beginning of the next word. (This, too can be turned off, although it’s hard to imagine why you’d want to.)

Although you don’t see it, the sizes of the keys on the iPhone keyboard are actually changing all the time. That is, the software enlarges the “landing area” of certain keys, based on probability. For example, suppose you type tim. Now, the iPhone knows that no word in the language begins timw or timr – and so, invisbly, it enlarges the “landing area” of the E key, which greatly diminshes your chances of making a typo on that last letter. Cool.

In the Safari address bar, you can skip the part above waiting for the Loupe to appear. Once you’ve clicked into the address, just start dragging to make it appear at once.

In bright light, the screen brightens (but uses more battery power). In dim light, it darkens. This happens because of an ambient-light sensor that’s hiding behind the glass above the earpiece. Apple says that it tried having the light sensor active all the time, but it was weird to have the screen constantly dimming and brightening as you used it. So the sensor now samples the ambient light and adjusts the brightnes only once – when you unlock the phone after waking it.

Incoming calls pause video playbacks. In this case, though, hanging up does not make video playback resume. Instead, the screen displays a list of videos. Apple says it’s a bug.

Remember that the speaker is on the bottom edge. If you’re having trouble hearing it, and the volume is all the way up, consider pointing the speaker toward you, or even cupping one hand around the bottom to direct the sound.

Ordinarily, the Contact list sorts names alphabetically, either by first name or last name. There’s no way to sort it by company name… or is there? Yes, there is. When you’re creating a contact, tap the First Last box – but enter only a company name (in the Company box). Then save the entry. If you bother to go all the way back to Contacts, you’ll see that the entry is now alphabetized by the company name. You can now reopen it for editing and add the person’s name and other information. The entry will remain in the list, identified (and sorted) by company name.

If you know that somebody has AT&T like you, add “(AT&T)” after the last name. That way, when he calls you, you’ll know that the call is free (like all AT&T-to-AT&T calls).

On the iPhone, buttons that mean “Save,” “OK,” or “Done” always appear in a blue box, where they’re easy to spot.