I'm not a researcher so I'm not qualified to make observations based on others (in theory not many people on here are) but I can use myself as an example of why I think txtin is not harmful to the English language.
Firstly, texting is one of many recently introduced linguistic deviations that have been accused of 'corroding' the language along with rising intonation (damn you Australia) and 'Americanisms' (God damn America). Languages change all the time and borrow heavily from other cultures, countries and subcultures when they do so (English is a prime example of this, we've stolen words from Latin, Indian, German, French, Russian, Greek, Gaelic, Arabic and many, many more), we do this for many reasons (practicality, new technology, social/cultural change, semantic and poetic value etc.).
I am very fluent in both standard English (along with some reigonal dialects) and text English (which I've picked up as I've used things like MSN, texting and online gaming) and I wouldn't say that the latter has impacted on the former. In my opinion, 'text speech' (or 'leet speech' for computers) is just a dialect or division of English that happens to be very widespread and understood, you don't critisise me for saying 'aye' instead of yes and I won't mention the fact that you write in shorthand.