Is Windows XP fully functional when installed using Boot Camp?

Yes. However, Apple notes that that the "Apple Remote Control (IR), Apple Wireless (Bluetooth) keyboard or mouse, MacBook Pro's sudden motion sensor and ambient light sensor will not function correctly" while running Windows XP. The Apple USB Modem, integrated microphone, and integrated iSight camera are supported starting with the Boot Camp 1.1.2 beta.

How do I select whether my Intel-based Mac boots into MacOS X or Windows XP?

Page 13 of the "Boot Camp Beta Setup and Installation Guide" explains how to switch between operating systems during startup:

1. Restart your computer.

2. Immediately press and hold the Option key until the disk icons appear on your screen.

3. Select the icon of the startup disk you want to use, and then click the arrow beneath the icon.

Apple also explains that:

You can use the Startup Disk pane of System Preferences in MacOS X to set the default operating system to use when your computer starts up. Boot Camp Beta also includes a Startup Disk control panel, installed in your Windows XP operating system, that works just like Startup Disk preferences. The Startup Disk control panel lets you set the default operating system when you're running Windows XP.

Can you "copy and paste" between MacOS X and Windows XP using Boot Camp?

No. Users who need this capability would be better served with a "virtualization" solution like Parallels Desktop for Mac or VMWare Fusion. With these programs you can "copy and paste" between MacOS X and Windows, just like you can using Virtual PC on PowerPC-based systems.

For more on these programs, please refer to the Windows on Mac Parallels Installation, Parallels Usage, and VMWare Fusion Q&As.