The more you hear about the defense, the more you hear about the 3-4. An example:
Ezeh said the team's defense under new coordinator Greg Robinson would mostly resemble a 3-4 ... One defensive player whose position has been adjusted is Stevie Brown. After playing (and often struggling) at safety last year, Brown will fill a hybrid outside linebacker-safety spot in the revamped defense
But I don't think it's an actual 3-4, by which I mean a Pittsburgh Steelers style 3-4 with big space-eating defensive linemen and havoc-wreaking linebackers. 3-4 versus 4-3 is often a proxy for a more fundamental distinction between defenses: one gap versus two. Wikipedia:
Sometimes the defensive scheme says he is responsible for only one gap - it's his job to make sure the running back can't come through his gap, and the other gaps will be someone else's responsibility. In this case we say the tackle is playing in a one gap defense. The tackle will line up right in the gap, not directly facing any offensive lineman.
In other schemes, the tackle will be responsible for two gaps. In this case the tackle will line up directly facing an offensive lineman, and his job will be to push that lineman backwards and make sure the running back doesn't run past on either side of his lineman.
If you want to play a two gap scheme, you need larger stronger defensive tackles who can control an offensive lineman or even two offensive linemen. If you want to play a one gap scheme you can use slightly smaller defensive tackles who are faster and more athletic and can penetrate into the offensive backfield more often. In a two gap scheme, the tackles are supposed to control the linemen, thus making sure that no one is blocking the linebackers behind them and the linebackers are then free to make the play and tackle the runner. So in a two gap scheme, you don't expect the defensive tackles to have a large number of sacks or tackles.
NFL style 3-4s are two-gap defenses with huge nose tackles and huge, mobile defensive ends who would probably be DTs in a 4-3. In college these sorts of folk are rare and the 3-4 is decidedly unpopular.
When it does emerge you get this a lot:
The picture is small, but that's USC in 2006. Note that the nearest defensive end is in a two-point stance.
You might recognize this defense because it's the one that blew up Michigan's zone stretch in the 2006 Rose Bowl. USC used Brian Cushing as a defensive end, but stood him up. This is basically a 4-3 defense with some extra frippery: you can back Cushing off or shift the line into another set of techniques or actually use it as a 3-4, or a 5-2 by moving the other linebacker up and loading the middle of the line:
That's what USC did in the Rose Bowl, putting one player on every gap and shooting it as soon as the line moved one way or the other.
So, it's a 3-4. Brian Cushing is technically a linebacker on the roster. But it's not a 3-4, as it's mostly a one-gap defense. In the NFL these days you're hearing a lot about "hybrid" defenses that shift from 3-4 to 4-3 based on situation, and that's what USC 2006 D was. They completely shut down Michigan's powerful rushing attack by shifting to a different front.
Given the wacky nomenclature of Michigan's Herrmann-era defensive line, which had a DE, an NT, a DT, and a "rush linebacker" I'm pretty sure that Michigan defenses from the era before I was paying enough attention to know were also flexible. This is what Robinson means by "multiple fronts."
Michigan is poised to go back to the future with three down linemen, two traditional linebackers, and a couple of hybrid folk. The "spinner" is a standup defensive end who plays on the weakside, and the "Stevie Brown isn't a safety" is a hybrid linebacker/safety sort who is probably going to function a lot like Brandon Harrison did under Ron English. Michigan had a specific spot they called nickelback; that guy always lined up over the slot receiver and blitzed a ton.
So it's going to look like a 3-4 at times, and there are only going to be three guys with their hand on the ground, and some announcers but definitely not Chris Spielman will talk about it as a 3-4, but I don't think it's really a 3-4. From my admittedly amateur perspective, it doesn't make any sense to take Mike Martin, a 290 pound penetrator, and turn him into a two-gap space eater. Nor does it make sense to take terror defensive end Brandon Graham and expose him to convenient double blocking on every play. Two gap defenses will probably be a changeup and nothing more. No matter how many people say it's a 3-4, don't believe them. It's not a 3-4.
Unless, of course, it is.