Leaving soft-money job for tenure track position that doesn't start until August: when to tell current advisor?
DO NOT say anything at all until your TT offer is signed, sealed and delivered. Hopefully this can be expedited. 

Once that's done, you should tell your mentor the information as soon as possible just so that s/he doesn't get the news from elsewhere. 


  The overall message you want to convey is: "I've enjoyed working here,
  and am excited to move on to the next phase of my career. I value the
  collaborations I've had with you so far and want to make this
  transition as easy for everyone as possible"


Don't worry about their reaction though: that's not something you can control, and for the most part a mentor will be happy for you (as well disappointed). Just make sure above all else that you are straightforward and open, and don't try to hide/shade/evade the truth for fear of "offending" anyone. 

Note: As regarding financial commitments, it might be possible to carry over some of the money to your new position, at least for travel/equipment/students. You should make a list of potential carryovers, and bring that up for discussion when you talk to your mentor.