Useful information about accessing a Mac OS X Time Machine backup disk on Linux: http://hints.macworld.com/article.php?story=20080623213342356
The relevant part being that if you look at a directory entry on the Time Machine disk, from Linux, you'll see something like:
-r--r--r-- 2155704 root 5791966 0 2007-06-25 02:54 Wallpaper
that first number, which is usually an indication of how many files are in a directory, is instead being used as a pointer to a directory named (in this case), dir_2155704 in the directory '.HFS+ Private Directory Data', which actually contains the files.
This information has proved very helpful over the last few days - although what I ended up doing was plugging the drive into a Mac and scp'ing the data across the network, since that was less hassle.
Looks like a good place for some Linux utility that provides a file browser for this kind of file system. Although I expect there's probably already one out there that I just didn't find yet.