This is growing to be a personal crusade of mine.
I may be horribly, horribly wrong, but I think it's clear that the Bengals cannot play a 3-4 defense this season despite all this talk.
There are only two reasonable 3-4 ends on this roster (Fanene and Robinson) and Thornton, while athletic, can barely hold his own as a tackle against the run. He is not a two gap end. Smith, Geathers, Rucker are not 3-4 ends. I'm not sure any of them would be successful as rush LB, much less true 3-4 outside LB. There is one 3-4 NT candidate on the roster - Shaun Smith. Sam Adams has never been and is not a great NT candidate. Size isn't everything. Peko, the DT drafted, could probably play a 2 gap role, but my reading of his scouting reports suggests he's a one gap player.
Thurman is a fit in any defense. Pollack looks like he'll be successful either way as well. Brian Simmons could handle a 3-4 ILB gig. Caleb Miller is a poor fit inside - he constantly battles to keep his weight up now. Landon Johnson could probably succeed if well protected by the line, but is probably too undersized as well. AJ Nicholson could work alongside Thurman if he keeps his head screwed on straight. Unless you see one of the miscast DE as a rush LB, there isn't a backer other than Pollack capable of holding up an a 3-4 OLB.
In my mind, this marks three straight seasons where the Bengals could have drafted much differently if they strongly desire to move to a 3-4 and haven't. Marvin doesn't want to reach for players, but it makes little sense to draft smaller linebackers and ends if your plan is to move to the 3-4. Fanene came in the seventh, Shaun Smith was a CFA. They've left guys like Chris Canty and Parys Haralson on the board in the middle rounds of the draft. Money concerns aside, they didn't even consider a player like Trevor Pryce or Kimo von Oehlhoffen in free agency.
Again, I am not a personnel man and Marvin clearly is, but I really think this should be a dead issue until 2007.
ETA: Smith is on his way out most likely no matter what defensive front they play. The Bengals will spend most of their money re-signing their OL. If there is any cap room left, they'll approach Smith. Makes gambling on Rucker all the more perplexing to me. As a fan, I'm hoping Eric Henderson shows he's healthy and forces his way onto the team as a CFA.