Bubble, bubble, toil and trouble.There may not be a hotter stock on Wall Street right now than Palantir Technologies. The company's artificial intelligence (AI) software is revving its growth engine, and there is a wide-open market opportunity in both government and the private sector. The stock is up approximately 800% since the start of last year.Be warned. Momentum can sometimes take stock prices to irrational places, and Palantir's valuation has arguably become untenable at more than 50 times the compan ...