MSDN had a sample for audit trails for 1.2, which was cool if you wanted to write your own code. In the event that you are like me and lazy**, you should look at this product.
Anyone with a need to audit or track all changes in MS CRM should look at VAST 3.0 from Visionary Software Consulting (www.vscrm.com). This add-on product lets you keep track of every change that is made in CRM 3.0, including changes to an unlimited number of custom entities. It also has a business intelligence component called VAST Insights that lets you query this historical data. You can see all your leads that haven’t been touched in a month, all opportunities that have been pushed out, etc. You can then pass the result set through workflow, so VAST can help ensure you follow up on every lead. All companies (particularly public companies, for SOX compliance) and industries will benefit from VASTs auditing capabilities, but especially government, financial, and health care, as VAST can help comply with regulations in those verticals.
I have done several demos with it and it is a VERY slick tool. The ability to see which fields where updated and when and by whom has helped out in a number of our deals that industry regulations require such a beast. :-)
Their website has a recorded demo and you can also download a version to work on Adventure Works Cycle.
** I don't do code. At all. Period. :-)