> Since 1.7 seems to be provided as "snapshots", does this mean that
> every user of 1.7 can have a potentially different version ?
> (depending on when they "happened" to fetch the sources)

Yes, I believe that is true. As far as I know, there was never (for example)
an official release of Ruby 1.7.1. It seemed to just "slide" into Ruby 1.7.2
at some point ;)

> Since the 1.7 snapshots seem to be done every night, I guess they
> can't all be equally "fairly stable" !??

I did just discover a separate snapshots subdirectory:

    ftp://ftp.ruby-lang.org/pub/snapshots

that may be worth investigating. There are Ruby 1.7 releases tagged with
certain dates, which perhaps indicates that they are more stable (whatever
that means ;) than the nightly snapshots.