<p>I've been developing Winforms applications in C# for a few years now and have been interested in moving future development toward WPF, mainly because of the positive things I've been hearing about it.  But, I'm wondering what sort of hurdles others have had to overcome as they migrated to WPF.  Was there a significant hit to your productivity or any particular issues which you found challenging?</p>

I'm not sure I can give you just one hurdle, because it is a complete departure from WinForms.  My suggestion is get Adam Nathan's WPF Unleashed, forget everything you know about building UI's with any previous technology (Winforms, MFC, Java) and begin again at square one.
If you try and do it any other way it will cause utter frustration.
ETA: the reason I say to just start from scratch is because sometimes it's easier to learn new concepts if you go in with a clean slate.  In the past, I've discovered that I can be my own worst enemy when it comes to learning something new if I try to carry knowledge from technology to technology (e.g. thinking that doing asmx web services for years precludes me from reading the first couple chapters of a WCF book).