I have been taking 100mg of Wellbutrin SR since my last trimester of my second pregnancy, and I was also nursing my daughter at this time. She would have been 13-15 months then. My daughter DID have weight gain problems and WAS diagnosed with FTT (and we were accused of neglect and forced into a whole series of major blood tests that left her in shock, but thats a different topic)---HOWEVER, my daughter's weight gain issues began before I started taking the Wellbutrin. The doctor first became unhappy with her weight gain at 9 months, and she continued to gain slowly, I began the Wellbutrin (at 150mg, later dropped to 100 just before the birth of #2) at 13 months, and around the same time was when we were forced to have the blood tests (which all came out negative for any problems, of course, my DD is just plain small). I do not think that the Wellbutrin has had anything to do with it. I continue to take it, and my son (#2) is small as well, and his gain slowed around the same time as my daughter's but fortunately picked back up again quickly. Thankfully we had switched to a ped who just said "feed him lots of fatty stuff".
At the time I began taking the Wellbutrin I called a LC I'm familiar with, and she said that her sources said that the medication in bupropion does come through in the milk, but that no babies in the test group experienced any adverse affects, so that would answer your question. Sorry I couldn't tell you the actual test or anything, I didn't ask for specifics.
My other thought is that having untreated depression could actually cause more problems for you anyway. I think I read somewhere that women with depression experience lower levels of prolactin, which may affect your let-down, and would definately affect your general mothering experience, making things a lot more difficult (I'm saying that from experience- I wish I could have those wonderful loving nursing moments I hear so much about). I'm not saying that Wellbutrin would raise your prolactin or have any affect on that, but at least it would take the edge off the emotional problems and make that end of things easier.
Also, get counseling too if you aren't! I hear so much about people who get prescribed anti-depressents from their regular doctor and then never get counseling. Counseling makes an indescribable difference, and it is possible to find counselors who agree with your parenting views, or even ones who may not, but will respect them and not ask you to change them. I go every two weeks to see a male counselor who talks to me for an hour while my 1 and 2 year olds nurse and wreck his office.