While learning something new is difficult it is also fun. If you are a professional or seasoned SQL Server 2005 DBA/Developer you have lots of materials and books at your disposal for learning SQL Server 2008. You obviously can't read and learn all of them but a good selection will make you at ease. I recommend anyone who wants to learn SQL Server 2008 the following titles which I found useful. Please feel free to comment and add any more titles you really think are useful.
Introducing Microsoft SQL Server 2008 R2 (Free E-Book Offer)
Accelerated SQL Server 2008 from Apress
Accelerated SQL Server 2008 from Apress
Pro SQL Server 2008 Reporting Services by Apress
Personally I love to read Microsoft Self-Paced Training Kit books for detailed knowledge and certifications. See below:
MCTS and MCITP on SQL Server 2008
For details please visit the following link to download the Certification Path Document for SQL Server 2008 designed and published by Microsoft. It is updated with required exams and recommended trainings and books as well as MCM and MCA certifications.