I'm using 800x600 in C# / XNA for compatibility with the XBOX 360, but would not bother with any less than 1024x768 for PC.
doesn't make sense...I've programmed pretty extensively in C#/XNA and XBOX360 can do plenty of high resolutions. 800X600 isn't even 720p.
But yea, it's rather dissappointing so many are still on 800X600. The current project I'm working on I hope to make resolution independent, maybe even include high-res textures if time permits (but this is touchy ground when you give users the option).