I think it's weird as hell when it is adults.
Yesterday was what I am told will be the first annual pajama day at work (on Christmas Eve Eve). We only have about 20 people in the office and probably 25% were off for the holiday and no one expected to visit, but still! Fortunately if we didn't want to partake in the adult pajama wearing we could wear jeans.
Not that anyone wore anything inappropriate, but I just really don't need to see my co-workers in pajamas.