DS is nearly 19 weeks and from day one he has dropped off while nursing. He's never been a great sleeper so I didn't discourage it but have worried instead about whether I was setting up a really bad habit - visions of him still needing to nurse to sleep at 6 came to mind. Especially since although he was able to soothe himself to sleep, he would only do so after a BF. Well.... last night he woke at 11.30 and soothed himself to sleep without wanting a feed first! Yippee!!!!! So, if you've been reading the stuff about setting up bad sleep associations, or been nagged at by your HV, then worry not!
(NB This is not intended to be a gloating post - just a reassuring one. I have spent/wasted so many hours worrying about this, and I don't suppose I'm the only one!)