I, for one, love a good emote. Just like a photo can illustrate a scene more quickly than actually describing it with words (in most circumstances), an emote can convey an emotion or a reaction more succinctly than actually writing it out. But is it ever acceptable to use them in a professional setting?
The short answer is yes, although this doesn't mean you should go about using :) willy nilly throughout your day.
Emotes are also useful in defusing a tense situation. If you and your coworkers are having a disagreement, or if tensions are high due to missed deadlines or user error (whether yours or someone else's), it might be good to throw in an emote to know that things are not quite so serious as they seem.
On the other side of the same coin, there are times you don't want to imply levity to your emails. For example, if you're delivering a serious memo on why Bob has not been pulling his weight the current project, you don't want to have your message diluted by a guy with his tongue sticking out. Another example is if you're sending out a company-wide email on any sort of company policy, skip the emotes. A catch-all rule you can use is that if anything in the message itself is negative, or is bad or serious news, or could illicit a non-positive reaction, skip the emotes.
Arik Hanson has a similar take for PR people that can be expanded to normal people. If it's a situation where you want to be taken seriously, be serious. Emotes are not serious, even if they are fun and useful in other settings.