In AST version 1.0.6 (?) Apple added the “Other NetBoot” option to boot other NBI:s hosted on the same server. This is great, except I think there's a bug that makes it incompatible with DeployStudio.
DeployStudio creates root images with the name “DeployStudioRuntime.sparseimage”. It also says so in the NBImageInfo.plist's RootPath key. For some strange reason, using the “Other NetBoot” command sets the root path to “NetInstall.dmg”. You can see it clearly by starting up in verbose mode or reading setenv. I suspect it's just a bug and that AST doesn't actually check the NBImageInfo.plist. I reported this to Apple, but didn't get a clear answer.
The result is you get a “stop” sign when you try to boot the NBI (machine gets the booter file via TFTP, but not the root image).
In any case, the workaround is simple - just rename DeployStudioRuntime.sparseimage to NetInstall.dmg every time you create your DeployStudio NBI with the DeployStudio Assistant and all will be fine. I just got bit by this the other day again and thought I'd write it down.