Well, if it were a big burden I would drop it but it really isn't blocking me from anything I want to do and it does not take a lot of effort (or at least I don't put in a lot of of effort testing and such).
Really there are only a few things developers should have to be conscious of to support Mono like case sensitivity of urls and proper handling of file system paths, ie using Directory.PathSeparatorChar rather than hard coding \ characters into strings, not using P/Invoke against native windows dlls (can't do that in medium trust anyway), etc.
Other than that if Mono is working like it is supposed to it should be the same as .NET on Windows.
What it would really need to stabilize Mono for mojoPortal is someone who is dedicated to testing and and filing bug reports to the Mono team and that is something I don't have time for and have not put much time in on for quite a while now. Without that I think it will always be hit or miss using Mono. I keep hoping that someone else in the community would be interested and committed enough to take charge of testing and reporting bugs on Mono. I would love for that to happen, but I'm far to busy to do it myself.