Applying to a lab as a post-doc; I am peer-reviewing their work I'm applying to a lab whose work I happen to be pretty familiar with, due to peer reviewing one of their articles.  

Is it ethical to mention this information to them?  I feel that if I don't mention it, and they find out later, they may feel that I mistrusted or misrepresented during the application process. 

Does the answer depend on whether the article has already been published, or whether it's still in the review process?
If you have previously peer-reviewed their work, you are obligated keep that information confidential unless the journal policy says otherwise.  If your potential employer somehow finds out you reviewed their work, they will know you did the right thing.

If you are currently reviewing their work and applying for a job with them, you should stop reviewing and contact the editor.  Tell the editor you now have a conflict of interest and cannot continue reviewing.