Just a few additional facts from sitting in at Mentor meetings on the issue of Mono with Babbage Linden and Periapse Linden:

Initially, the rollout of scripts on Mono will be kept voluntary - options will be added to allow residents to opt into Mono when compiling their scripts, at least until LSL on Mono works exactly the same way as the original LSL2 implementation (excepting speed and efficiency issues)

At some point however, there will obviously be no real need to keep things running on the original LSL2 implementation. Babbage has expressed that as much as possible, the final removal of non-Mono LSL will NOT break currently available scripts and migration will be done in a way that gets people noticing only because of accelerated speeds and extra room for code. or because of the original uncompiled script being lost, as noted by Tat.

Just my L$2 worth of notes.