[_delirium]

It depends on what level of compatibility we're looking at. It's true that MIT-licensed code can migrate to a GPL codebase but not vice-versa. But a different kind of compatibility (what I was thinking of) is whether you can maintain separate and separately-licensed codebases and legally distribute the combined binary. If DTrace were MIT and the Linux kernel were GPL, you could legally link them and distribute the result, so in that sense they'd be "compatible". Which isn't the case if you have a CDDL and a GPL codebase.