When I bisected this issue between latest and 2.6.23 the issue seemed to be
here (round about 2.6.24-rc1). Trying to repeat the same things on latest just led to confusion.
So I've started from scratch with contemporary trees and found something very different.
2.6.25: works
2.6.25.5: works
2.6.25.6: broken
2.6.25.10: broken
2.6.26: works
Now the issue which was introduced in 2.6.25.6 is that a rdtsc will be done
even though this CPU does not support it (before console_init(), leaving
me with only one led for output!). This issue has been fixed in 2.6.26 but not in 2.6.25-stable.
The protected mode jump was just a red herring (or at least whatever was broken back then is fixed now); 2.6.26 is working fine and that's good enough for me.