I may be making too much of it, but I think this could mean two things.
First, this could illustrate George's skepticism about who Clarence is. He doesn't fully accept his claim to be an angel, and this visual divide between the two represents the barrier of George's unbelief.
Second, this separation of George could act as a foreshadowing of the world Clarence will show him where he's never been born. He's set apart visually to prefigure his absence from Bedford Falls.
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I also noticed a slight but significant difference between George's two interactions with the man whose tree he runs into with his car. Right after George hits the man's tree, the man complains to George and says, "My great-grandfather planted this tree!" After George wishes that he'd never been born (and thus hasn't run into the tree), he encounters the man and comments about his tree, and the man mentions that it's "one of the oldest trees in Pottersville." In both, the man is concerned with his tree, but in the world with George, his concern is more personal (related to his great-grandfather), while in the world without George, his concern is centered on the status of the tree itself. Such a difference in the man's concerns seems to suggest that in the world with George, a greater emphasis is placed on personal connections.
