Q:

Switching to WPF.  Is it time? 

I'm considering switching from MFC to WPF. 
My first concern is that there are too many users who don't have .NET with WPF installed yet.  Can anybody point to a source containing the WPF penetration numbers? 
My second concern is speed.  
Any other considerations? 

A:

I've been banging away at WPF for a while now.  It is brilliant, but it still has (occasional) holes you've to plug yourself.  However all indications are .NET 4.0 will be a significant step forward. 
I would say start now.  The WPF learning curve is REALLY steep, and it'll be a while before you'll be releasing software to users, believe me.  Also do yourself a favour and get the WPF Unleashed book.  It's superior. 
Speed isn't a consideration.  The power WPF gives is well worth any drawbacks with speed, which - coming from Windows Forms - I haven't noticed to be honest. 

A:

What kind of application are you developing?   If it's a wide-distribution desktop app that you want your grandmother to install, your concern about .NET 3.0/3.5 adoption is valid.   So far from what I've seen, performance is less of a concern.   

A:

WPF penetration
First of all, Windows Vista and Windows 7 both have WPF preinstalled, which accounts for 35% of the market automatically.  Windows XP has had it as it had .NET Framework 3.0 as an option in Windows Update for over three years, and many applications ship with it, so it is likely to also be installed on a high percentage of Windows XP machines.  StatOwl indicates that about 80% of NET Framework installations are version 3 or above. 
If you're shipping on CD it is no big deal to include the latest .NET Framework on the CD and have it install automatically.  If users are downloading your application, it can contact Microsoft's web server to download and install the latest .NET Framework.  Online ClickOnce deployment also has this capability if you want people to be able to start their application directly from the web browser without installing it. 
So the bottom line is, you probably don't need to worry about whether people will have WPF installed on their machines or not unless your target market consists primarily of dial up customers on Windows XP who don't run much third-party software (i.e., they just run Windows and your application). 
Speed
Not an issue.  I have a 200 MHz Pentium Pro with 384 MB RAM from 1998 that I test my software on, and my WPF applications have comparable performance with equivalent MFC applications.  If your WPF application uses lots of fancy graphics and animation it will run slowly on ancient CPUs and graphics cards, but so would an ordinary MFC application with the same features.