Windows Phone Trick: Add Emoticons to your Contacts
If you want to add a little something something to your contacts, we ran across a way to add emoticons to your contact's name over in the Windows Phone Central Forums. All you need to do is start a text message and add the emoticon of your choice.
From there, highlight and copy that emoticon. Leave the text message and go to your Peoples Hub, find the contact and choose to edit the name. Add a space at the end of the last name and paste the emoticon. It's a neat way to add a little customization to your contacts.
Edit: As noted in comments by hopmedic, to prevent it from breaking voice dialling, put the emoticons in the last name field, but put a hyphen after the last name, and then the emoticons.
Thanks, danclyons, for the tip!