Reply to self:
so I downloaded the flasher 2.0 at work to a USB memory stick. At home at my private notebook, I booted Windows, copied the flasher and the big binary image to C:/ (just to find it more easily), and renamed the image file to OS2006.bin in order to avoid typos later at the command line.
Then booted Linux (at the PC), started a console as root, went to the WIndows partition, and followed these instructions: http://maemo.org/maemowiki/HOWTO_Fla...ImageWithLinux
Did not work, even as root I did not have the permissions. Thus I copied the two files from the Windows partition to the Linux partition, and executed the flasher 2.0 calling the file I had renamed to OS2006.bin including the appropriate parameters.
And: just as promised, it worked.

Conclusion: it's not yet fool-proof, if you don't have the latest software. But taking into account Nokia's focus on consumer products, I assume (and hope) that they will be able to provide a simpler way to upgrade from any platform. Ideally (OS2007?) : using ONLY the 770; download the next binary image to the mmc, having a flasher app running on the 770, and rebooting from mmc. This would avoid all issues with Mac, Linux, Windows, Vista(?) flashers.

Side-track: thanks for the private message; no I most likely don't spead viruses by not having the service packs; I manually installed all patches directly from Microsoft while this had been possible, and I have other security tools, and a secure DSL router.
It's about control: I want to be able to decide, at least in principle. To use an example from my industry (pharmaceutical): Patients participating in clinical studies before the drugs are approved by FDA or other authorities have not studied medicine, they are not experts; nevertheless, they formally agree to the treatment. They are in a weak position, compared to the doctor or pharmaceutical company, but they collaborate based upon the notion of "informed consent". We have to make sure that they understand what is going on. Which is ethically O.K. and different from attempts in the Software industry to disempower the individual by legal (endless EULAs and other conditions) and other (e.g. secretly installing spyware) means.

That's one more reason why I like Nokia's approach. In my case, it's not about free = zero costs, it's about the other meaning of free.