Interestingly, only three linebackers (four if you count Orakpo) were selected in the first round of the 2009 draft. Three of those players made the Pro Bowl. Brian Cushing, Brian Orakpo and Clay Matthews (injury replacement). The other linebacker was Aaron Curry.
In my opinion, Denver must decide on who'll be their starting quarterback and starting wide receiver opposite Eddie Royal. The Broncos have Kyle Orton (who isn't the longterm answer), Brandon Marshall and Tony Scheffler as free agents. They'd drafted Robert Ayers last year, so they might not draft a DE/OLB in the first round again.
As said, Miami recently signed Karlos Dansby to start with Channing Crowder at inside linebacker. The Dolphins resigned Chad Pennington as a backup. Davone Bess, Anthony Fasano and Jason Taylor are free agents. And they just cut Gibril Wilson and Joey Porter.
San Francisco makes the most sense because they've two first round picks and can afford to take a chance on McClain. The 49ers can use depth at wide receiver and corner back with Isaac Bruce, Arnaz Battle, Dre' Bly and Marcus Hudson among others as free agents. They could trade for a starting quarterback if Alex Smith isn't the longterm answer.
Pittsburgh could start McClain with Lawrence Timmons as veteran James Farrior is getting older. In my opinion, the Steelers need depth at safety and corner back with the injury proneness of Troy Polamalu, and William Gay, Ryan Clark, Deshea Townsend, Keiwan Ratliff and Tyrone Carter are free agents. Probably, could use a backup quarterback as well. Charlie Batch is a free agent.
I don't see any of the non 3-4 teams selecting McClain. Again, it'll be either the 49ers or the Steelers selecting him in the first round. If he's there for the Patriots, I don't think it'll matter if history dictates that they'll pass on him or trade down. Because if McClain is valued higher than players such as Sergio Kindle, Dan Williams or Jared Odrick. Then they'll select him... Because McClain at pick twenty-two equals good value. In other words, you didn't have to reach for him.