Since most of the communication we practice is via email, the time zone difference causes no problems at all. In cases when the clients are US based it is easy to establish a very productive workflow when they contact us at the end of their work day, which is the start of ours. This way the projects are in constant progress 24/7.
Upon the clients wishes, we are able to set up skype meetings. There are a lot of time zone converters online, which come in handy when trying to arrange a call.