A few notes for some of the people that have been posting in this thread:
1. aMSN does support video with iSights (I often use it with my external one) but doesn't support audio. If you want to do video conferencing to someone with a PC I recommend using aMSN + Skype, it works great for me.
2. Windows on an external disk isn't as great as it sounds. OS X installs can often be used without any problems in another computer with different internals. This even worked with the transition kit installations. Installing OS X on an external hard drive on a DTK and then putting that hard drive in an old iMac G3 works. The new Intel versions of OS X will probably not be backwards compatible with PowerPC machines due to the use of EFI and a different partitioning table, however, switching hard drives between MacBook / MacBook Pro / Mini / iMac and Mac Pro should work as long as OS X is updated to the latest version that was shipped with the Mac.
Windows involves hardware extraction layers and gets very messy if you try to use it in another machine. If its an identical machine it should work and it should also work for Mini to MacBook or iMac to MacBook Pro because they're very similar. A MacPro to any of them will cause issues. All the drivers will be wrong.
So, simply put, being able to install Windows on an external disk would be fine for keeping your Mac innards seperate from Windows entirely but it would be useless for using that install of Windows on various different computers.
