Also on Mac in in 4.1.0 (build 45).
File - New Solution - NUnit Library Project
Added 1 line to the pre-generated code and set a breakpoint on it.
"Run Tests" works, but for "Run Tests With ... Mono Soft Debugger" the Test Results Pad opens up, then nothing more happens. It just stops.
NB The only UI I can find 4.1 for debug unit tests is in the Unit Tests Pad, right click and choose "Run Tests With ... ". Have I missed something?
NB: in other solutions I find unit test debug sometimes works and sometimes doesn't.