It's been discussed as much as, if not more than, Green Jerseys and who's going to start at QB in '07.
The discussions should predominately be in the Football Forum under Defense, Travis Thomas, Minter, etc. I tried a search but the search function doesn't like "3-4" or "34". I did find one thread visually
http://www.irishenvy.com/forums/showthread.php?t=16208
Preseason in discussing the Travis Thomas to LB move, Weis was asked in a press conference (around Aug 13th) about using the 3-4 considering the size of Thomas. Implying that Thomas would do better with 4 LBs than 3.
Weis replied to the contrary that "Size matters in a 3-4, it doesn't in a 4-3". He amplified his reasoning in the press conference. There might be a transcript I used to post them (but I didn't look for it tonight).
From what I recall of the many threads on the topic that while it's popular with fans (whatever scheme we NOT using is popular with fans)
ND doesn't have the horses up front on the DL nor at LB. As ND rebuilds through recruiting ND may move in that direction in time. Keep in mind if ND got 3 DLs and 4 LBs in the next class, ND would not open with a 3-4 as those rookies have to build their bodies up to play against 21 and 22 year old OLs that have been lifting and developing technique for 3 or 4 years.
Weis has consistently says he plays what the talent lets him play (offense or defense). We don't have the horses yet and the few we have are ponies right now.
Feel free to identify the Chris Zorich clone in the middle of the DLine and the 4 LBs like Stonebreaker, Dubose, Kowalkowski, and Jones.
I have trouble finding one LB of that quality, strength and experience on the team. Check out a Depth Chart from those days. ND was 3 deep at all 4 LB positions and there was a solid 2 Deep on the DLine. We can't fill a first string today.