I was playing New Vegas this morning, working on getting at least one of the story branches completed and the game began to hit some lag around twenty minutes in. I fixed it, somewhat for a minute or two, by switching to the dashboard and back. But two hours later when I went to the dam to do the 'You'll Know It When It Happens' mission the game lagged so badly that it crashed during the mission and corrupted the auto save that had been made just before the mission began in full.
The game lags heavily in The Strip and the dam, hopefully a patch down the line will fix that, makes it really hard to get anything done in those areas.