Good news: the Portal 2DLC hasn't been delayed. Instead of moving it out of its "summer" release window, Valve is releasing the DLC as late in that window as possible. Speaking to Kotaku at Gamescom, Valve marketing director Doug Lombardi said that the DLC, which includes new maps, leaderboards, and challenge modes, will be released in mid-September. That's "still technically summer," he noted.
If you're upset about this DLC arriving later than you expected it to, but still within its stated release window, let us remind you: this is more Portal 2, that Valve is giving you for free. "Thank you" is pretty much the only appropriate response to that.