Ext4 support for Macbook / OS X

You could use a FAT32 formatted partition. Both OS X and Linux can read/write that format. If your hard drive is partitioned using GUID, then you can resize an existing partition and make a new one to use for file moving. Format the new volume MSDOS (FAT32.)

The main drawback of this is when you copy OS X documents to the volume. FAT volumes strip a file's Resource fork creating two files on the FAT volume - the file and the file's Resource fork. This will create some problems with some files - they may become unreadable or lose formatting. To avoid such a problem zip the files before putting them on the volume. A bit of a hassle, but will generally prevent problems from the different filesystems.

Not likely. However, you could install Linux on a separate partition.

I forgot about them. They also have a similar solution for NTFS.