Wal-Mart, the world's largest retailer, has announced flat profits of $3.02 billion and slightly lower sales of $93.5 billion, down from $94 billion, for the first three months of this year compared with a year ago.   For the current quarter, the world's biggest retailer forecast results roughly in line with analysts' expectations as it faces tough comparisons with a year ago, when it got a boost from customers spending tax rebate cash in its stores.  "We're pleased to report that fiscal year 2010 is off to a very good start," said Wal-Mart chief executive Mike Duke.