For Silverlight developers, Expression Blend is old news, however, for most developers Expression Blend is a foreign tool. If you are an iPhone or Android developer, you are probably very focused on getting your arms around Visual Studio 2010 and may not know that much about Expression Blend.
I can tell you flat-out that Expression Blend WILL make your life easier as a Windows Phone 7 developer. By far, the best way to learn Blend is to watch someone use it, either in person or via a screencast or webcast because Blend is very visually oriented. Up on the Design site off of Microsoft.com there is an incredible amount of content available on design and Expression Blend at this link:
Spend time with this training and you will absolutely save time in the long run and may even end up with an improved User Experience in your Windows Phone 7 applications:-)