Quickbooks 6 for Mac can READ Quickbooks 2003 for PC files -- it converts them automatically to a Mac format file. You can also save your QB6 Mac file to a QB2003 PC file.
However, you can not read and write the same file transparently (at least not using version 6 of Quickbooks). They know they have to do that for true cross-platform functionality, but it didn't get there in the Version 6 release.