I understand WMV is a microsoft format. To view it on a Mac, you need Microsoft's Windows Media Player program. If it is ever possible to view WMV on the iPad, it may require an app from Microsoft.
I found a free program called HandBrake that runs on the Mac. It can convert several video formats including WMV, into something that can be played in iTunes on the Mac, and played by iPad's Video program when synced. However, this would be of little use for WMV attachments in iPad Mail.