I've been considering experimenting with game development and XNA. I'm already an expert C/C++ programmer. I read through some C# books, but haven't done any development in C# yet.
What's a good resource for learning XNA, from the point of view of someone who's already an expert programmer?
Check out the XNA homepage and the tutorials over there, under Community -> Resources. As an experienced programmer you should be able to take it from there.
For more in-depth infos browse the XNA Team's blogs, also linked from the XNA Creators page.
This is good: http://www.riemers.net/, just keep in mind that navigation is through the bar on the right. I must have been tired because it took me a while to figure it out :-O
I'll second reimers and the creators.xna.com samples as a good way to get a handle on how to quickly whip things up.
On the other side of the spectrum, I highly recommend Nick Gravelyn's Tile Engine tutorials. It's a different approach, as the entire series is presented in video. It seems like a great place for beginners to get started, though new coders might have a bit of trouble with his pace. Having said that, the section on the Content Pipeline (which is an XNA-specific implementation of the pipeline concept) is a good introduction.
Ziggyware also has a good selection of tutorials, some of which are more advanced.
Shawn Hargreaves, one of the XNA's devs, has a great blog that let's you in on the internals of XNA a little more. Check out the archive if there's a topic that interests you in particular.
I would say that a library called XNA Debug Terminal should be of some help to you. It is open source and can be setup in seconds. It allows you to see the value of any variable, invoke any method, watch values changing in real-time, and more by simply typing c# code into a terminal-like display that appears atop your game window. Unlike the normal Visual Studio debugger, you can invoke arbitrary code while your game is running. You can find out more about this at http://www.protohacks.net/xna_debug_terminal . This will greatly help you to avoid a lot of frustration while learning XNA.