Ralph Mason posted to the Mono list about his frustration with the lack of progress on a debugger for Mono:
This single thing will give the most major boost to mono of any possible code that could be written. Mono is a great piece of work, but it's being totally let down.
One of the replies was the usual open source argument of "Why don't you help out and try and contribute code to make the debugger better?". I think that is missing the point. A lot of people who might be interested in developing their .NET code on Linux don't have a compelling need to do so, and are not likely to want to spend a lot of time working on a debugger to get to that point. If it is in the interests of the Mono project to increase usage of Mono then, as Ralph says, it is better to direct effort towards the debugger than version 2.0 of Mono.
The good news is that Paolo Molaro replied to say that work will soon be starting in this area.