Spring certainly sprang this year when it came to residential real estate values. For the second month in a row, home prices increased from April to May. The S&P/Case-Shiller 20-City index showed on Tuesday that 19 out of the 20 cities it tracks registered price gains for May, including Minneapolis (2.8 percent) and Atlanta (2 percent). Las Vegas was the only city in the index to file a decline, with home prices now 56.4 percent below their August 2006 high. Vegas is now a gamble in more ways than one.