Error 29 Solution
After a 24 hours fight with my 3GS here is the solution for problem 29.
In my case, problem 29 came from the fact that my battery wasn't a "Apple" battery. So no way to upgrade and your Iphone will become bricked if you try to upgrade. Worst thing is that it never happened with the other updates.
All you need to do is to remplace your battery with your original one (In my case the battery was faulty during a travel so I had to go for a generic one that worked better than the original one - So I had to take my other Iphone battery to make it work).
Make sure to have the latest ITunes software (9.2) and that's all!
Don't bother to try on another computer or restore older version as it didn't work for me.
All the best!
IPhone, iOS 4, Windows 7