Despite the move to iTunes Plus for new songs, the free song of the week has been offered in 128kbps DRM format. Songs of the Week are presented as a single-song album. What I have discovered here in Australia, for at least the last month, is that the free song has also been available in a 256kbps DRM-free version. Instead of getting the free song, click the artist name in the “breadcrumb trail”:
Look for an album with the same artwork as the free album. Ensure that it is an iTunes Plus album:
In that album will be the same song but in 256kbps and also free. Get that version instead:
I guess it will take a while to transition away from the old free song arrangement. Perhaps these have been decided on months in advance and so we still see them until they run out.