It’s not quite ready for use in routine cases, but according to the Independent, it’s been successful in helping prosecute one chap:

The approach was used in the case against David Hodgson who was convicted in February of murdering his former lover Jenny Nicholl, 19. Ms Nicholl’s family kept receiving messages from her phone after she had disappeared, but police suspected her former boyfriend was sending them, not her.

“A linguistic analysis showed that text messages sent from her phone were unlikely to have been written by her but, rather, were more likely to have been written by Hodgson,” Dr Grant said. Hodgson had a different texting style to Ms Nicholl. For example, he would write “I am” in his messages, while Ms Nicholl would use “im”.

That has to be rather tenuous though, does it not? ‘I am’ vs ‘im’? It’s hardly reliable when you take into consideration T9 or your handset playing up. For instance, sometimes I write my text messages in English. Other times I use txt-spk. Or sometimes a variation of the two.