Applying for another job within same department? I've been a postdoc for just under a year in a nice lab, and I have no problems with my boss or colleagues. However, another position has just come up in the same department, which fits way better with my career plans. I've been advised to apply anyway, and only mention it to my boss after an interview / offer are made, if I get as far as that. The reasoning being, there's no need to cause awkwardness in the group if I'm not even shortlisted. The problem is, the HR persons responsible for recruitment are the same for both departments. So, even if I ask for confidentiality in the application form, it's not impossible that the information could leak to one of my colleagues.

So the questions are:
 - From a PI's perspective, how acceptable is it to hear one of your postdocs is trying to move to another job in the same department?
 - From a postdoc's perspective, what is the less damaging strategy in this scenario? Try to keep it confidential, and risk the info leaking to your PI? Or telling your PI about it before you apply, and risk creating an awkward feeling among your group - especially if you don't get the job?

Disclaimer: I'm aware of previous questions around the theme of multiple job applications to the same University / department; and of questions about changing departments while in a graduate program. What I'm asking is neither of those, as you're already employed and thinking of moving within the same department.
This seems to me to depend on personalities more than anything. You need to make a judgement about the reaction of your boss. Most people, IMO, would support you in any career advancement. Giving early notice, in most situations, is beneficial, as it lets them know that they may need to search for a replacement or otherwise arrange schedules, etc. 

Putting your options out, as you have here (fantastic opportunity, career goals...) would probably be a good way to approach it. I don't think anyone expects you will remain a post-doc forever. 

If your boss is a reasonable person, then it would seem to me that s/he would be happy for you, and happy that if you get the new position that you will still be available for consultation and that your expertise won't be lost. 

There are exceptions, of course, even beyond personality. If one is working with highly sensitive information, for example, it might be necessary to escort them off premises immediately on hearing that they might be leaving. But the situations in which you would need to hide your application would, to me, be rare. 

Finally, I think that the issue of same department has no real world meaning in such a decision. For example, most people would celebrate a move from post-doc to tenure track faculty.