Most speakers that connect through the 30-pin port on the bottom of modern iPods also fit iPhone. You may need one of Apple's Universal Dock adapters—a white plastic booster seat that makes most iPod models sit securely in speaker docks—for a good fit, especially if you have an iPhone 3G. (And frankly, external woofers and tweeters sound infinitely better than the iPhone's tiny, tinny speaker.)

One major thing to remember, though: electronic interference. If you forget, the iPhone will remind you. If it senses you're seating it in a non-"Works with iPhone" speaker system, you'll see a message suggesting that you put it in Airplane mode. Doing so takes care of the interference, but it also prevents you for making or getting phone calls. You can blow past the warning and keep Airplane mode off, but you may get some unwanted static blasts with your music.

Those little gadgets that broadcast your iPod's music to an empty frequency on your dashboard radio are a godsend for iPodders who don't want to listen to the same 40 songs over and over on commercial radio. Unfortunately, these transmitters are not so hot for the iPhone. Again, electronic interference is an issue, unless you put the phone into Airplane mode. Transmitters that connect through the headphone jack, meanwhile, probably won't fit the first-generation iPhone.

If you've ditched your telltale white iPod earphones for a higher fidelity headset, you probably won't be able to connect it to the Original iPhone's sunken headphone jack—at least not without an inexpensive jack adapter from Belkin and other manufacturers. Thankfully, the second-generation iPhone 3G has a normal unrecessed headphone port; you can plug just about any regular headphones or earbuds into it.