OK what happens for me very specific...
I will select a thread in SMS to respond to, and sometimes the phone (I have an HTC EVO running baked snack 1.8 with the stock SMS app) will select a DIFFERENT thread, and if I do not notice, then I will have typed out a message and it goes to the wrong recipient.
If, however, I DO notice which thread is brought up (about 1 in 10 misses, and it's not just fat fingers, it's a thread from MUCH lower down in the list, not next to the one I intended to select) and I bring up the correct one, it sends to the intended recipient correctly.
Hope this helps.