As someone who has written a simple O/R Mapper - Hayden.ActiveRecord - I can tell you that writing a simple, opinionated O/R Mapper is easy and fun! But writing a good, performant, flexible, pluggable, feature-rich O/R Mapper takes a heck of a lot more dedication and skill.
However, I remember quite a few fun evenings and weekends putting together my O/R Mapper which is why I like to read and poke through source code of others doing the same. On CodeProject you can find a recently submitted Lite ORM Library Article talking that discusses someone's attempt to create his own O/R Mapper with downloadable source code. I dare say that it is missing a lot of functionality that you would expect from a real-world O/R Mapper, but this is how we learn and get better. If you are a budding O/R Mapper developer it certainly couldn't hurt to play around with the code.
I might recommend another open source O/R Mapper, Retina.net, to check out that has recently come to life on CodePlex-
You can download the source code for Retina.net. It appears to be much more feature rich than Lite ORM Library.
I have a category on O/R Mappers that spans quite a few O/R Mappers for quite some time. As much as I hate writing production data access code, I never tire of talking about it :)
Check out Lite ORM Library v2 here.
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