I had exactly the same problem and found the solution quite by chance...the machine (Latitude CPx H500) would take quite a while before starting post, it would literally just sit there for about 3-4 miunutes then give me a prompt the hit CTRL-X to exit Manufacturing Mode. No matter what I tried (including all the suggestions in the sorum), it made not difference.
Then I had an idea, maybe there was a problem with the power board. I changed it and all my problems were solved. The machine boots right up and gives no Manufacturing Mode message.
I have to admit, I'm not sure why the power board was causing this wierd behavior but for the $20 it cost me for a repacement board on eBay it was well worth the money!
Hope this helps others. Feel free to email me at email@example.com for further details and let me know if this helps.