I run into this about once every 18 months.. my customers are all pretty smart and 'don't do dumb stuff' like this but inevitably some company acquisition or rogue division that lived outside of the jurisdiction of central IT presents this scenario. Because we don't have a consistent story across every single product I can never remember the correct answer to the inevitable question "Can we upgrade the evalaution version of SQL Server 2005/2008/2008 R2 to a licensed version without having to re-install?". Like a goldfish that is (allegedly) always pleasantly surprised by the new surroundings of the same old fish bowl, I'm always pleased to find that the answer is Yes, you can 'upgrade' an evaluation version of SQL Server to a full licensed version (staying on the same version number of course).
Version and Edition Upgrades
Process to upgrade:
How to: Upgrade to a Different Edition of SQL Server 2008 (Setup)
How to: Upgrade to a Different Edition of SQL Server 2008 R2 (Setup)
Upgrade from SQL Server Evaluation Edition to full blown editions? (SQL Server 2005)
In 18 months time when I get asked again (although hopefully for Denali rather than SQL Server 2005), hopefully my Bing search will find this page!