Sharing information openly with customers empowers them to engage with your product or service in a more intimate way. It gives them more options such as self-service. It helps them understand your product and service. Consider this: the more secrets you keep, the more you alienate other people.
Lenovo is a great example. For every product they make, Lenovo freely provides a highly comprehensive Hardware Maintenance Manual. The PDF-based HMMs shows you everything you could want to know about your computer - from replacing the hard disk drive to the LCD panel itself, complete with detailed line drawings of every component. Sort of like IKEA assembly instructions writ large and without the ambiguities. As an example, this is the HMM for my Thinkpad T60p.
A conventional, fear-based approach could be "No way, we can't possibly do that. This sort of information will just encourage our dumb users to take apart the products and break them." Lenovo's approach is positive and empowering. It says they trust their users with real information; and that they would use this information wisely and appropriately. For those users comfortable with a screwdriver, they can choose to replace a hard disk themselves. For users with unsupported older models, they can even learn to service their computers themselves.
I just spent 20 minutes and $9 from Dick Smith to replace the backup battery in a Thinkpad TransNote, saving myself over $270 had I simply ordered a new part. More importantly for Lenovo, it helped me bond with their product (yes, I know, this sounds so geeky and sad LOL).
(I have documented the process here if you are interested.)
So, what are you keeping secret that you need not?