Tons of computer users have experienced the same problem as I did. They installed Google Earth, created lots of bookmarks for places before they exited Google Earth and got the messages that the myplaces.kml file could not be written down; writing to myplaces.kml.tmp instead.
I haven't found a functional fix anywhere on the Internet but it's so simple.
Go to the directory with settings, something like the Windows 7 location in C:\Users\Luboš\AppData\LocalLow\Google\GoogleEarth. The remarkable thing you will almost certainly find out is that the myplaces.kml file doesn't exist at all. That's why it cannot be rewritten, and the command is only able to rewrite it, not to create a new one.
So when Google Earth is turned off, grab the myplaces.kml.tmp file you find there, press F2 for rename, and erase .tmp from the file name. Or find another method to rename it if this one doesn't work. The file should acquire a nice icon representing its association with Google Earth and you will never experience a "could not write" error message again.
I suspect that the same fix works for other operating systems, too.