I had the same problem you describe happen to me yesterday and today while driving home. I plan to take my car into the shop either tomorrow or early next week. In the manual it says to call lexus immediatly if your ABS and brake light come on while driving. I have heard it could be anything from just low brake fluid, to replacing a master brake cyclinder. I'll keep u posted with what my local mechanic tells me.
//Update
After reading a few more post, it looks like we should check are tire pressure level. It may be low due to the cold weather....I just got my tire replaced 3K miles ago at costco. I plan to go there first thing tomorrow to have the tire pressure checked.
