The question is clear from the title.
Is there an SDK for the Java Micro Edition available for Mac OS X or Linux? Or does one need Windows XP to develop JME applications?
No, with a 'but'
There is no WTK (Wireless Toolkit) from the main vendors available for Mac (NetBeans Wiki reference) so basically you can't debug nor test the code. But, you can use an emulator from a third co: MPowerPlayer
If you are using netbeans in any other OS but windows, you still can download and add the J2ME packages using the tools/plugins menu. Then you can add the MPowerPlayer to the IDE as a J2ME platform. Netbeans (and i guess any other IDE with J2ME WTK support) will recognize it as a valid WTK,
I've posted an specific entry about this in my blog, but it's in spanish, although maybe you can follow the images. Give it a try, and don't hesitate to ask anything: J2ME in Mac Os with NetBeans