I would make the distinction between pushing your employer to acknowledge your value to them and actually coming out and saying you're actively thinking of leaving.
In my consulting career I used every appraisal meeting as an opportunity to reinforce that I saw myself - as did colleagues - as amongst the top performers of my peer group; so I was happy with my situation all the time I saw myself being fast-tracked and rewarded accordingly.
This sends the message that you are one of the team and don't want to be in a position of ever having to look elsewhere, but you do expect to see decent career progression and your managers do need to be pushing on your behalf in order to be sure of retaining you.
In this scenario you are perceived quite differently from someone who is actively voicing their discontent and making it known they are looking for new opportunities. As per the other comments, once you are perceived as someone who is heading out the door the incentive to have you staffed on the best projects and to be pushing for raises and promotions on your behalf is hugely undermined. So I would agree with the other comments that if you are going down the route of applying elsewhere then you don't discuss this at all until you have a contractual job offer in hand - at which point your employer may choose to counter offer.
Hope this helps and good luck with progressing your career.
Tony Restell
Top-Consultant.com
