People generally think emoticons are a professional no-no, but I think it depends on your industry and relationship with the contact. The occasional smiley face could work sometimes when you're trying to soften a sentence or expressing gratitude and enthusiasm. However, it's a good idea to use the happy emoticon sparingly and not plaster it all over your emails. It might just be me, but I think the other emoticons, such as the sad or frowny face, are slightly more unprofessional.
What about you — do you use smiley faces in professional emails?