Last year, I posted on how to use Enterprise Library 3.1 with SharePoint under partial trust. It was incredibly difficult to get it to work with SharePoint when using partial trust because none of the DLLs had the AllowPartiallyTrustedCallers attribute on them. This meant in order to get it to work, you had to add it to every AssemblyInfo.cs in the solution, sign it with your own strong name key, and then build and deploy it. It was a lot of effort.
Enterprise Library 4.0 has been out for a few months now, but I have yet to comment on it. I am pleased to report that with this new release, none of this overhead is required. All of the DLLs, have the AllowPartiallyTrustedCallers attribute which means all you have to do is reference the DLLs and assign appropriate permissions. I recommend putting Enterprise Library DLLs in a solution package which will make setting permissions easier. Last year's post should have most of the settings that you will need.
If you got turned off trying to implement Enterprise Library in the past with SharePoint, you might give it another try. With the APTCA change, it should make it much easier to use entlib for logging, exception handling, data access, or whatever.