I'm currently developing on a 64 bit machine. I have never developed on such a machine before. Does the fact that I'm on a machine like this require that set some compiling flag to compile to code that will execute on a 32 bit machine?
No. At compile time, compilation is done to java bytecodes -- which is not dependent on the bit size of the native machine.
At run time, however, it depends on the type of JVM. If you are using a 64 bit JVM on your 64 bit machine, the JIT (just in time) compiler will compile the bytecodes to native 64 bit instructions. If you are using a 32 bit JVM, on either a 32 bit or 64 bit machine, it will still JIT to the native 32 bit instruction set.