As the N900 is a true multitasker, sure this is possoble. Just press the upper left icon to bring the active tasks up. if contacts are not open already, click in the area without a window to bring up your desktop an select contacts or click on the icon up left in the taskview to bring up application overview. you can do whatever you want if you leave the phone-application running in background. if you are done, bring up taskmanager agan and select the phone app.