I just tested it, to see if I could post a picture on twitter if I texted it to it, and I refreshed the page, and it had posted my phone number!!
I can't remember quite what it said, I just saw my number and thought OMG what the hell!? and deleted it pronto!!
It said something like '02 number 07******** has sent you a message...' the something else
Texted tweets is ok though! Just if you don't want your number on there, don't do what I did!
I've had no dodgy texts yet.. so it seems to be all good!