> Even if 1.9.3 is still binary-compatible with 1.9.1, I think that it would be easier to change

Easier to whom?  You?  You know I have compilers so it's 100% OK for me to
recompile my extension libraries, but I think that's not for everyone --
especially for Windows users.  Even if a Windows user had a compiler (rare!),
recompiling an extension lib is painfully slow on that arch.  So it's not
always "easier".

I know it makes _you_ at ease and I'm not against your motivation.  I hope
there is a good trade-off.

> - enable users to co-install (1.9.1 or 1.9.2) and 1.9.3. In debian, 1.9.3 could be provided as a separate ruby1.9.3 package.

Why do debian have to provide a distro-standard way to have multiple ruby?
That is, why you have to retain 1.9.2 installations once after you have 1.9.3?

> - advertise the fact that there are API changes in 1.9.3 that could break apps

Is this, if any, intentional?  I think such breakage is rather a bug.