Yes, it's possible. I have one in my pocket wherever I go.
Boot time is okay, but the initial installation takes a bit fo time due to the generally slower write times of flash drives.
Just format it with the GUID partition scheme with one HFS+ Journaled partition and use the Snow Leopard install disk to set it up (if you want it to boot more than one model of Mac,dont' use the software disks that come with a Mac, use the retail Snow Leopard disc). Run all the software updates after.