Most recent S&P/Case-Shiller Home Price Index (just released for February) indicates that we are currently at the April 2009 lows as 20 out of 21 (except for Detroit) surveyed metro areas have experienced home price decreases. This means that if we have another month of index declines, technically we'll be in the "housing double-dip." This doesn't necessarily mean that we will have a double-dip in the overall economy and that U.S. GDP will decline, but growth is certainly likely to be further subdued as consumers' confidence wanes.
Declining home prices combined with sustained high oil/gasoline prices can definitely create an environment susceptible to a substantial sell-off in equity markets in the summer/fall timeframe, depending on when evidence of this pressure on consumers will become evident to markets. It's unclear how light summer volume will contribute to market volatility: last two summers have presented us with a largely sideways action although, oddly enough, during both there was an uptrend in May/June, sharp drop-off in early July, and strong uptrend through the fall. More likely that not this is a coincidence, but considering mounting pressure on the consumer, this sell-off could very well be triggered during the same timeframe.