MonoRail & ActiveRecord tips
One thing I would add to the ActiveRecord tips is be mindful of lazy-loading on collection mappings. Especially now we have DetachedCriteria and the ability to be explicit about fetching strategies. Lazy-loading big lists can be pretty disastrous to application performance when used in the wrong place. For example: when traversing every node of a persistent object graph and touching unloaded collections.
Take a look at the NHibernate performance docs for more information.