Hi all. Thinking ahead slightly, we have a Moodle install whose sole job is to network (with MNet) our main Moodle and seven Moodles from local schools.
We are planning to upgrade to Mdl 2 in December 2010, but for one reason or another the seven schools probably will not. (We have no control of their systems, they pay for 3rd parties to maintain and so on).
There should be no trouble networking Moodle 2.0 and Moodle 1.9 together. However ***please test it thoroughly, well in advance** (for maximum points of helping the development team, do it while Moodle 2.0 is still in beta )
Having said that, the new MNET test client you find in the administration -> development tools item in the admin block in Moodle 2.0 will not work properly with Moodle 1.9. However, this is just for developers, so shouldn't affect you anyway.