This is a bit of a pain, so I made this little applescript to avoid having to dance with the preference panel every time. I just compile it into an app and save it on my desktop (or in the scripts menu) to be easily run whenever the desktop turns blue. Sometimes I have to run it twice, but that's easy too.
tell application "Finder"
set currentPicture to the desktop picture as string
set the desktop picture to ¬
":Library:Desktop Pictures:Solid Colors:Solid Aqua Blue.png"
set the desktop picture to currentPicture
end tell
If anyone has a real fix instead of this workaround, I would love to hear it.
[robg adds: I noticed this very thing this week while at Macworld; after both of my sessions, when I removed the projector, my desktop went to the standard Aqua blue background...]

