The best way to learn is to just grab the new product and go for it.
If you have an MSDN account you can download SQL 2005 from the MSDN site. If not you can buy the Developer edition for about $50 US. SQL 2008 is still in beta so you can download from the Microsoft.com for free.
Other than that, asking questions and reading the forums is the best way that I’ve found to learn the new products. Especially with SQL 2008 where there isn’t any real documentation about it yet.