I'm a developer and don't want to commit to iOS4 yet as it's buggy, but a few of my clients will be early adopters. Can I use OpeniBoot (or something similar) to boot two legit OS's (preferably 3.1.3 and iOS 4)?
Short answer: No.
Long answer: No, because Apple put in checks for this after people did this with 1.1 and 2.0. They only want to be on the first partition, so you can't have two OS versions on different paritions. Theoretically, you could patch this out, and have two partitions, but it's going to be extremely difficult and probably cheaper to just buy a second device.