OK here's the latest situation with GPS-CS1 support on Macs, following the release of Mac OS X 10.4.9.
If you have an Intel Mac, then the 10.4.9 update will add GPS-CS1 support.
If you have a PPC Mac (G4, G5 etc) then it won't.
It appears that the USB kernel extensions have been updated in the Intel version of Mac OS X, ever since a slightly newer version of Mac OS X 10.4.8 came out with Core 2 Duo Macbooks. 10.4.9 gives these updates to all Intel owners.
The key extension is probably IOUSBMassStorageClass (as the GPS-CS1 should mount as a USB mass storage device) which is at version 1.4.5 on the PPC code, but at version 1.4.7 on the Core 2 Duo Macbooks and now on all Intel Macs running 10.4.9
So if you're an Intel owner, congratulations!
For those of us still running PPC Macs, there are three possible hopes.
1. Try to lobby Apple to do something about this. There is clearly a code change that can be made to fix this problem (it isn't a hardware problem, I can mount my GPS-CS1 on my iMac running Ubuntu).
I raised a bug on Apple's bug reporter system last December and have sent updates whenever we've discovered something new - e.g. the Core 2 Duo support, the fact that the Intel version of 10.4.9 fixes this.
However Apple have not responded beyond automated acknowledgments and the bug status has never been updated. You might be able to help draw their attention to it by sending an email:
Email email@example.com and ask them to look at bug number 4898558
Even better if you live near an Apple Store, you could try to visit it's Genius Bar and bring this blog and the bug to their attention in person.
2. The source code for version 1.4.5 of IOUSBMassStorageClass is identical for Intel and PPC. If we can get hold of the source code for version 1.4.7 then presumably it can be compiled for Power PC, which we could install ourselves as a custom patch at our own risk. This will probably be released on Apple's Open Source site at some point (I've now added it to the Useful Links section). It took about 6 weeks from the release of 10.4.8 before the source code went online on that site.
3. As Leopard will apparently be a very substantially rewritten version of Mac OS, then perhaps the USB libraries will be updated for Leopard. Then again as Apple don't seem to be aware of this bug, it's possible that any USB updates will actually break GPS-CS1 support for Intel owners again.
As ever I'll keep everyone posted.