Personally I think Sony has enough brand power that MS can't quite go all out on a price war for fear of looking desparate. MS is better off matching price and using the extra money to fund franchises that will sway PS gamers to their console. If the Xbox ever achieves the same mindshare/branding that PS has, then MS can go in for the kill and lower pricing such that Sony can't afford to be in the business anymore.
That probably won't happen in this generation, mainly because Japanese studios are still going to favor the PS3 for development due to the fact that MS has no traction in Asia. That means that PS3 will have a few key franchises like MGS, FF, DMC etc... that should stop MS from dominating.
You also have to remember that Sony's 1st party division is still better than MS', until such time that MS can churn out more hits than just Halo, Fable and Project Gotham Racing. GT3, God of War, The Getaway, TM, Wipeout, R&C, J&D, Sly, etc... have all been pretty well established in the minds of gamers. Rare is the big wildcard. Can they bringout Kameo, PDZ, Banjo, etc... to rave reviews and fill all the holes in the 1st party lineup? That's why MS acquired them, but only time will tell...