I assume you are referring to SyncML?
Yes.
Apple does have an iSync plug-in development tool.
I know. I use it daily creating iSync Plugins for many phones.
Does it even really require SyncML to work?
Yes it absolutely does. SyncML is a fundamental requirement for the iSync Plugin maker to be able to create a plugin for any phone. Read here:
http://developer.apple.com/documentation/DeveloperTools/Conceptual/iSyncPluginMa kerUserGuide/Introduction/Introduction.html
http://developer.apple.com/documentation/AppleApplications/Conceptual/iSyncSyncM LGuide/iSyncSyncMLGuide.pdf
http://developer.apple.com/documentation/AppleApplications/Reference/iSyncSyncML Ref/iSyncSyncMLRef.pdf
Couldn't the plug-in translate between whatever format the data is received from the phone to SyncML?
No it can't. In any case, what would the point in that be? SyncML is an industry standard for transferring data between devices. Why go to the effort of reverse engineering proprietary protocols for every phone when standard ones exist? That is exactly why the SyncML standard was created, so in theory, any device can talk to any other without reverse engineering protocols.
If bitpim can do it, then it must be possible.
BitPIM doesn't translate to SyncML. It just reads the phonebook data directly from the phone using it's own code. They have to reverse engineer each individual phones communications protocols then incorporate that code into BitPIM every time a new phone comes out.
I have a feeling it's more likely that LG engineers just don't use Macs.
No. It's because LG haven't incorporated the industry standard SyncML protocol into any of their phones, except the new KM900 Arena, for which now an LG produced iSync Plugin exists.