Apple includes a 10 volt USB wall-charging adapter with the iPad
that extends about 3 feet. For a little extra freedom, Apple sells
a $30, 10 volt power adapter for the iPad that runs 6 feet long.
The 10 volt distinction is important, since most USB power adapters work at half that voltage and are only half as effective (or worse) at recharging the iPad in a timely fashion. Using a 10 volt charger, a spent iPad should recharge to full power in around 3-4 hours. On a 5 volt adapter for an iPhone or iPod, a recharge could take all day.
To make a long story short: the voltage of your iPad power adapter matters.
For in-car charging, look for adapters that advertise 2.1 amp USB power, instead of the more common 1 amp sockets. A product like the <="" a=""> offers both types of USB charging on a single adapter.