could not modify my previous post anymore, weird.
Some more info : I changed the command subkey from :
"D:\work\cb_snap\src\devel\codeblocks.exe" "%1"
to :
"D:\work\cb_snap\src\devel\codeblocks.exe" "test.cpp"
So I could check if DDE would fall back on this, it seems NOT, since I still get the error message on the file I double clickd (which wasn't test.cpp ;-) )
[EDIT] : nope I was wrong, CB started but the clicked file was not opened any more, why because it now wanted to open the test.cpp, which did not exist, so in the end it did fall back on the command subkey.
But just before this fallback we get this error message, i wonder who gives this : DDE/Windows or CB ?
Now the following is also rather unclear, comes from :
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/shell/programmersguide/shell_basics/shell_basics_extending/fileassociations/fa_verbs.aspifexec. Set the default value of this subkey to the DDE command to be used if DDE conversation cannot be initiated. When initiation fails, the application specified by the default value of the verb's command subkey is launched (SO IS IT THIS 
?). If an ifexec key exists, its default value will then be used as the DDE command. If there is no ifexec subkey, the default value of the ddeexec key is used again as the DDE command(OR IS IT THIS
??).