As newly minted princess Kate Middleton greeted Michelle Obama in the ornate surroundings of Buckingham Palace Tuesday, it was hard to believe she was wearing a $340 dress from British retailer Reiss — a frock any commoner could buy. Middleton’s appearance in the nude-hued “Shola” dress sparked a shopping frenzy, causing the retailer’s Web site to crash and the dress to immediately sell out. And now style watchers are hailing the Duchess of Cambridge as “the pauper’s princess” — the first stylish royal to embrace affordable trends, and the perfect recession-era symbol.