As opposed to Per's super advanced PowerShell scripts here's a very good introduction to a bit more advanced PowerShell concepts than just the fundamentals -
There are many excellent introductory materials covering the Exchange Management Shell in Exchange Server 2007. This article aims to enforce the fundamentals with practical and useful examples. The goal is to help an Exchange administrator better his understanding and command of the EMS in performing routine messaging and related tasks, freeing up time for more important tasks.
Find the article here. Seen on the writer's blog.