As promised here's the update for the progress on my installation of Xubuntu Jaunty Jackalope on my G3 iBook.
The basic problem was this, I have an old 128Mb USB flash stick, but it won't mount on the computer. I had had the same problems when running an earlier version of Xubuntu on my iBook and I had thought it was to do with USB Flash disk support or FAT support on PowerPC / Apple hardware. But this time, because I really like Jaunty Jackalope, I really wanted to get it working.
It's possible you've found similar problems. You plug in a USB Flash drive and it doesn't mount. You can check what it should look like when you mount by sticking in your Xubuntu installation CD (I had to type eject at the terminal to get the tray to eject) - it should pop up in the Thunar file manager window.
But with my USB Flash drive that didn't happen. After some investigation I found that the program dmesg (which logs debugging messages from the kernel) had this log at the end:
[98622.283596] sd 1:0:0:0: [sda] 256000 512-byte hardware sectors: (131 MB/125 MiB)
[98622.293940] sd 1:0:0:0: [sda] Write Protect is off
[98622.293964] sd 1:0:0:0: [sda] Mode Sense: 00 00 00 00
[98622.293972] sd 1:0:0:0: [sda] Assuming drive cache: write through
[98622.345699] sd 1:0:0:0: [sda] 256000 512-byte hardware sectors: (131 MB/125 MiB)
[98622.362888] sd 1:0:0:0: [sda] Write Protect is off
[98622.362912] sd 1:0:0:0: [sda] Mode Sense: 00 00 00 00
[98622.362921] sd 1:0:0:0: [sda] Assuming drive cache: write through
The Mode Sense bit is where it first goes wrong. It wasn't to do with file system support, because it was never able to read even the first sector. I found out from some Linux bug lists that some USB Flash drives simply don't read in Linux. I've got a good hunch as to why. It's because the testing procedure for most Flash drives probably isn't that rigorous. They test the hardware with Windows (and maybe Mac OS X) and as long as it works there they're happy.
The solution is to try another USB Flash Disk. I tried a friend's and hey presto, it worked!
So, I then tried connecting my Nokia N95 via USB and that worked, here's the image.
So, the moral of the story is: try another Flash Disk!