If you are reading the excellent NHibernate FAQ Blog you're probably using a System.Guid as id for your entities. Unfortunately if you are planning to use NHibernate.Search to enable Lucent.Net queries against your entities this won't work. NH.Search will throw a:
"Unable to guess IFieldBridge for System.Guid" - Exception
After some debugging (a great way to learn new frameworks) I found the problem. NH.Search has no "bridge" that can convert a Guid to Lucene format.
I got it working by creating my own GuidBridge. This is why I love OSS-projects. If you find a problem you don't have to wait months for someone else to fix it for you!