In a surprise appearance at Prince William's Royal Air Force flight-training graduation ceremony April 11, Kate Middleton never looked more like a princess-in-waiting.

Seated near Camilla Parker Bowles and Lady Sarah McCorquodale (Princess Diana's sister), Kate, an onlooker told PEOPLE, "was beaming" as her beau of five years and 32 others got their wings from one of Britain's Air Chief Marshals—who happens to be Prince Charles.

Smiling broadly after the ceremony in Lincolnshire, the normally camera-shy couple sent speculation about a long-awaited engagement into overdrive. "Got Something to Smile About Kate?" teased the Daily Mail. Mind reading aside, royals expert Judy Wade says Kate's presence at such a public event—her first since William's graduation from Sandhurst academy in 2006—signals that an engagement "is inevitable. They might as well be married."

Indeed, a friend says that after a brief split last spring, William, 25, and Kate, 26, have seemed more solid than ever. "It's a case of when," the pal says, "not if."