First off, let me say I very much like working with Spring both on the .NET and Java platforms. The functionality provided truly makes working in either platform much nicer!
I just did get some hacks in place to successfully get Spring.NET (core) working in OSX (10.4.6) with Mono (1.1.15). Below are the lines changed in order to accomplish this. By no means am I stating that these are "good", "proper", or "correct". However, they worked for me and maybe they'll help out a fellow Spring-er.
In Spring.Core/Core/IO/FileSystemResource.cs, line #265:
// Changed from using the Uri property to use "fileHandle.FullName" property
return new FileStream(fileHandle.FullName, FileMode.Open, FileAccess.Read, FileShare.Read);
In Spring.Core/Objects/Factory/Xml/XmlParserRegistry.cs, line #73:
// For whatever reason, registering the validation parser causes Mono to bomb!
// Commenting out this call enables core to work (after a fashion)
// new ValidationConfigParser(),
These are the only changes I can recall making to have Mono on OSX run the Core. Unfortunately I've not gotten the test cases running yet, however these changes do enable a couple of "hello world" type IoC sample projects to work.
Keep up the great work on spring and I'll post more results/mods/experiences on the OSX/Mono platform as they are relevant.