I've heard only good things about virtualizing Mac OS X on VMware Fusion. Have you personally tried it? Of course only the server version can be virtualized according to the EULA.Can I install Snow Leopard and Lion on my i7 Mac?
Yes you can.
Do I need to segment the drive?
Yes since they are different operating systems they each need their own partition.
Can I do it without that?
No, unless you virtualize one of them using Parallels tools or VMWare fusion but I wouldn't recommend it as the experience will be very buggy and there will be a lot of lag.
Keep in mind, if you have a newer computer (Late 2011 MacBook Pro or newer MB Air) you are unable to install Snow Leopard as the computers shipped with 10.7 and you can't install anything earlier than what came on it when it was new.
Can you use an external hard drive with Snow Leopard on it as a boot disk if you have Lion on your Mac?
Keep in mind, if you have a newer computer (Late 2011 MacBook Pro or newer MB Air) you are unable to install Snow Leopard as the computers shipped with 10.7 and you can't install anything earlier than what came on it when it was new.