My RT ticket #3824 was finally filled late last week, so I should be receiving my OpenMoko-based (developer preview) mobile phone sometime in the next couple days.
As far as getting Mono functional on top of the device, it seems that the OpenEmbedded project and their BitBake build tool.
I'm wondering how easy the device will be to develop for in terms of getting Mono, IronPython, etc running on top of it, but if it really is the fully-featured Linux-based phone it's reported to be, this could turn out to be lots of fun (regardless of whether or not I can actually use it as a replacement for my miserable Cingular phone).