The Walking Dead does totally steals off 28 Days Later...
But 28 Days Later stole that aspect from The Day Of The Triffids
28 Days Later came first... way first... just because the release dates are close to each other doesn't mean a thing since 28 Days Later was being filmed in early 2001.
Confirmed by this tidbits on imdb.com
"The shot of the notice board at Piccadilly Circus, with the missing persons flyers, created a degree of controversy when the filmed was first shown, with some saying it was insensitive to what was happening in New York after the 9/11 attacks. However, the film was shot prior to 9/11, although it was released after it. In any case, Danny Boyle has stated that he based the shot on a photograph he had seen from an earthquake in China, but he also acknowledges that had he made the movie post 9/11, he would not have shot the scene."
So, yes, The Walking Dead stole the opening sequence. But then again 28 Days Later takes inspiration from other stories as well, confirmed by screenwriter Alex Garland.
From imdb.com
"Scriptwriter Alex Garland acknowledges several sources as inspiration for his screenplay, notably John Wyndham's The Day of the Triffids, George A. Romero's "Dead" trilogy (Night, Dawn and Day) and The Omega Man. Direct homages include Jim waking up in the hospital from The Day of the Triffids, the chained infected being studied from Day of the Dead, and the scene in the grocery store (people in the mall from Dawn of the Dead), the stop for supplies that saw a run-in with infected children (also Dawn of the Dead (1978)), and the military holing up against the plague with outsiders partially to deliberately include females (also Day of the Dead)."