I ve somehow managed to make my desire unrootable. I rooted it with an android 2.2 rom (dont remember which!) then one day i formatted the SD card. I 've tried every possible combination to root the phone again but nothing works. Also, because the build i am using is "passion", unrevoked3 refuses to root it for me.
You can change the "passsion" flag. it's in the /system/build.prop file. Open it as root and change it to what is right. (check for the correct value in the file from the cyanogenmod release)
Otherwise, just install alpharev and kiss that partition limitations HTC put goodbye. (you can flash permanently any recovery you like.)