A bit late but for the benefit of those that across this. Follow the following steps. Example say you want to break lines for "Hello World" between the two words. Then go to note pad and type the following.
concat("Hello","
","World")
Then copy this to the insert formula section in the default value of the field in the infopath form.
This should do it. What you do it you add the invisible line break character to the concatenation by pressing Enter between the " and ". I have not yet tested this in the browser though. Hope this will help.