For one, it can be 'date' in chat speak.
Second of all, it can be a smilely with D as the mouth and 8 as the eyes. Not many people know what it means, but I believe it may be an extreme case of D:
Girl texting boy: 'hi, i lyk u! wana D8?!"
Person 1: *Cough cough* I'm dying... x_x
Person 2: No!! D8 Don't die!!