i was having the same problem.. it was during the transition of 10degrees to 40degrees.. maybe it was what you said.. all the road grime.. but at anyrate.. it has seemed to of stopped sometime within the past couple days..
just let everyone my 2005 G6 gt has 13000 kms on it and i had all the rotors replaced because they were warped when stopping at highway speeds and the brakes still squeak when it is cold for the first 2-3 stops, ............
as a licensed mechanic for 14 years i can tell you that GM has changed the make up in the material of there brake pads and thats why they are squeaking, my Envoy is doing the same thing and it is at the dealer right now getting new pads and rotors all the way around(8000 kms on it)...................!!!!!!!!!!!!!!!!!!!!!!!!!
I was at the dealership today for the squeaking brakes and the whistling sound on braking too. Got the pads and rotors replace today took the whole morning I was glad they replace them and not just say we can't do nothing about this.