As of today, the Mono build in OpenEmbedded is working pretty well.  Mono support for ARM and other embedded architectures should add some interesting capabilities for embedded Linux devices.  Having written a number of large embedded Linux applications, I can really appreciate the advantages of high level languages for exception handling.  It is also interesting to note that Java support in OpenEmbedded is making progress as well (http://jalimo.org/wiki/doku.php).  Windows CE developers have already taken to the .net compact framework in a big way, so its nice to see similar capabilities for embedded Linux systems.  

A feed has been set up for installing mono packages on your Openmoko phone.  Add the following line to a /etc/ipkg/*-feed.conf file:

src/gz mono-armv4t http://dev.bec-systems.com/feed/openmoko/mono

And then:

ipkg update

ipkg install mono

The packaging is fairly granular, so look at the listing in the above URL to see a complete list of what is available.