in windows 7 beta there is a boot to vhd feature which allows you to start a vhd and may avoid the old school option of dual boot. however with windows vista there are a couple great things with dual boot.
1) if you dual boot windows vista with an earlier operating system, when you are in windows vista the partition with windows vista is still the c: drive and the prior operating system will use a different drive letter.
2) if you are already running windows vista, you can dynamically resize your partition allowing dual boot with either a server operating system or client operating system. the possibilities this provides are endless. dual boot between 32-bit and 64-bit on the same system. install both windows vista and windows server 2008, install windows vista and windows 7, etc.
i shared the repartioning tip last week and i don't think many people were aware of it (especially through the gui in windows vista - if you are running windows xp, you have to use diskpart from the windows preinstallation environment - windows pe)