Unfortunately it will not always work (sorry for the bad English) like in my case
The Problems also occur if there are some corrupted OS files in the System
So what you need to do in this case is (3 step work)
1.grab OS X install DVD or USB and boot from it
<IMPORTANT after the boot is done DO NOT FORMAT THE HARD DRIVE OR YOU WILL LOSE ALL YOUR DATA. I WARN YOU!!!!>
2.click on the Hard drive you want to install the OS X
3.click install and wait it takes longer than usually
the Install Program looks for OS X on the Hard Drive !!!
If there is OS X the Users Data will not get touch only the Systems data gets Updated (10.6>10.6.7) or Downgraded (10.6.7>10.6)
so don´t worry about your Data the are safe
(I done it today Downgraded my System 10.6.7>10.6 without a Backup but I don´t Recommend it. ALWAYS DO A BACKUP!!!)
all my Data still there and settings like i had never touch the system great
and know I can boot back into the System without "waiting for DSMOS"
hope it can help other people who has the same problem