It was Howard who realised that Sheldon and Penny had become 'neighbours'.
It was when Penny came over and asked for a cup of sugar. Sheldon stood, moving to the kitchen, asking if that was white, brown, caster, or icing sugar that she wanted? He then proceeded to rattle off the pros and cons of each variety. Howard sat on the couch and realised as he watched that the two had become more than just people who lived across the hall from each other. He observed as they made small talk, Sheldon painstakingly measuring out the sugar, and Penny asking how his latest experiment was going. He replied and handed her the cup, careful not to spill any. She smiled and thanked him, and he nodded politely before walking her to the door, wishing her well with her baking or whatever it was she was doing. She smiled again and said that she'd bring over some cupcakes for them later. He said thank-you in advance and ended the exchange with a polite smile.
Howard thought that it was a very good thing that they had adjusted to each other so well. Though they still bickered, it was nice to know that they were at least capable of being good neighbours to one another.