Robert, while not common, it has actually been documented multiple times in the U.S. alone. A presentation given at the Delaware Valley Ornithological Club showed several examples, including a fantastic shot of a Rose-breasted Grosbeak caught at a banding station one spring.
John, if you do a Google image search for "gynandromorph bird", you get a few interesting examples including more than one Cardinal, a Rose-breasted Grosbeak, and an Evening Grosbeak. Interesting that 3 big beaked birds that are relatively close in heritage pop up. Makes you wonder if there's a group more prone to this than other, though the program had a photo of one warbler.