Look at your AC adaptor for the phone to see what its current rating is. My Samsung Galaxy S phone charger is rated for 700 mA.
Most cigarette lighter adaptors put out up to 1 amp so they are fine. I have several different manufactures' cigarette lighter adaptors and they all work fine on my Galaxy S phone. Typically, I see an unloaded voltage of about 5.5 volts on these adaptors. This allows for some cable and connector voltage drop. Drawing 1 amp though a micro USB connector is a stretch.
Some phones (like Motorola) rely on external charger module for current limiting. This reduces the power dissipation in the phone's internal charger regulator by allowing the phones internal series pass device to go into fully ON switching mode. It is not absolutely necessary but the phone will back off on charging periodically to avoid too much heat build up in the phone if phone regulator has to operate in linear mode. It will just take a bit longer to charge the battery.
Also, phones must be happy if the USB 5v source limits the current to 500 mA since this is the maximum spec on USB devices. Some USB ports, like on laptops, or even phones with USB On-The-Go capability, limit sourcing to 100 mA from their USB output. Most desktops allow more then the 500 mA limit.
Some portable cellular data modems, and external portable disk drives come with a USB cord with two USB connectors. Since these devices need more then 500 mA to operate, they put two connectors to theoretically get 1 amp supply by using two USB ports. Some laptops will not allow two ports to simultaneously draw 500 mA each, also there is no guaranty that the each port might have independent 5 volt regulators that have a little different voltage due to regulator tolerance. This causes the slightly higher voltage port to take most of the load. Some laptops will shut down the port if an overcurrent limit is detected.