If you’ve upgraded to Lion from Snow Leopard and you can’t live without your PPC-only apps, you have a couple of choices. One answer would be to partition your disk and install Snow Leopard as well, allowing you to boot between the two operating systems by holding down the ‘option’ key when you power up. Alternatively, if you don’t want to use up your internal disk’s precious space, you could install Snow Leopard on its own dedicated external disk. Have a look here for instructions.

A different answer could be to run Snow Leopard concurrently within Lion using virtualisation software such as Parallels or VMware, though there is some question about the legality of this move (more on that below).

Note that I haven’t tried this myself, but the word around the community is that it does work. Take a look here for guidelines on how to get going, or follow this excellent guide here.. If you have Parallels and your SL install disks already, nothing to stop you from giving it a go. If you don’t have it, you can download the trial and test it out for free. If you decide to keep it, you’ll need to pay for Parallels when the trial expires.

Now, about that legality issue. In the past, the End User license agreement that Apple supplied with its OS disks prevented you from doing something like this. However, the new agreement under the App-store downloaded Lion states that you are allowed:

to install, use and run up to two (2) additional copies or instances of the Apple Software within virtual operating system environments on each Mac Computer you own or control that is already running the Apple Software. <— source

Now that doesn’t necessarily imply you’re allowed to run SL under a virtualisation machine. This agreement comes with Lion when its downloaded from the App Store, but its unclear whether it refers to all iterations of OS X or just the latest one. That’d be one for the lawyers I guess, but it is probably safe to say that so long as you’re using licenced Apple software and authentic Apple machines, you are unlikely to incur Apple’s ire. That said, always remember that what you do with your computer and your software, you do so at your own risk!

Related articles:
Macworld: VMware Fusion update lets users virtualize Leopard, Snow Leopard