This is assuming you did not change the folder names and let Clockworkmod create them. I haven't had it happen until today, but it's not a nice message to see when you are trying to do a restore.
Code
Checking MD5 sums...
MD5 mismatch!
To fix this problem use the code below. Substitute the final folder name for the backup you need to fix.
Code
adb shell
# cd /sdcard/clockworkmod/backup/2010-06-29.20.22.53
# rm nandroid.md5
# md5sum *img > nandroid.md5
When that finishes, re-run the restore and everything should be fine.
NOTE: There could be something really wrong with your nandroid, so use this tutorial as a learning experience and not a fix-all. In my case, everything was fine after restoring. However, that may not always be the case. Do this with caution.