I'm going to disagree.. With the chance of the CBA not being done in time (limiting the time with player), and the 49'ers, Dolphins, Cowboy and Brown's GM all staying the same, I'm not seeing it happen this year..
What does the GM have to do with it? Let's look at the GMs:
Cowboys - Jerry Jones - Has been the GM since the late 80s and hadn't ran the 3-4 defense until Parcells got there in the mid part of last decade. He isn't married to it.
Browns - Mike Holmgren - Never ran the 3-4 as a head coach. He is a 4-3 guy. Part of the reason why Mangini was fired was because he doesn't run Holmgren's system. He is likely to hire Gruden who came from Holgren's tree and runs a 4-3 defense. Odds are very good that the Browns will switch to a 4-3 next year.
Dolphins - Jeff Ireland - Part of the Parcells regime and could get canned. Owner Stephen Ross is going after a superstar head coach (Jim Harbaugh, Jon Gruden). To get one of those guys, he is going to have to give them free range including determining what type of defense they run. You don't get a marquee head coach and force him to adopt a system they don't want to run.
49ers - Trent Baalke - The 49ers made him GM last week. He has been with the 49ers since 2005, but it is unclear how married to the 3-4 he is. They are also pursuing Harbaugh too. Like Miami, to get Harbaugh they will have to allow him to run the system he wants to run.
At least one of the four teams will almost definitely switch to a 4-3 (Browns). Two others might depending on who is hired since they are pursuing marquee names for head coaches who will most assuridly have total control. Of the four teams mentioned, the only one that is very likely to remain a 3-4 team is the Cowboys considering they are keeping Garrett for at least the 2011 season.
The question will be will Jim Harbaugh adopt the 3-4 like his brother (who doesn't use it exclusively and runs a lot of 4-3) or will he stick with the 4-3.
Cowher, where ever he goes, will definitely implement a 3-4 defense. Denver with John Elway now the GM may be looking for a coach that might not run the 3-4. So they might change. So it those could be a wash depending on where Cowher goes (he could go to a team who already runs the 3-4 like Miami).