No worries about the current official DOSBox binary not running under 10.5. At least the developer pre-releases of 10.5 are known to execute it successfully. There is just a little notice that one of the OSX APIs in use is deprecated and shouldn't be used anymore.
Compiling DOSBox under 10.5 could be an issue though. At least with the current developer releases of Leopard quite a few of SDL based OSS projects report troubles.
So for now the solution is to use the official DOSBox binary under 10.5 until a special Leopard version is out. This will probably be at the end of the year - maybe earlier. 😊