As mentioned, the consumer Snow Leopard license does not allow running in a virtual machine, only the more expensive server Snow Leopard license allows running in a virtual machine.
However, you can boot under Snow Leopard, and then run Lion in a virtual machine. You could give Lion the "Lion Share" of your system's resources, and then spend most of your time in the Lion virtual machine. When your PowerPC dependent app has been replaced with something that runs on Lion, you can switch over to full time booting Lion.
I know it is not really a great way to go, but if you really need this PowerPC app enough, it might be worth it. Or just stay on Snow Leopard until you no longer have a PowerPC code dependency.