MGMAT indicated in one of its guides that the correct idiom is "concerned with" (SC 2007 guide - page 111). But I have also seen "concerned about" as correct. Can someone explain what the difference is and when to use each?

"concerned with" means involved with or connected to

"concerned about" "concerned at" and "concerned by" all mean worried about

So correct usage would just depend upon the intended meaning.

Also FYI "concerned + preposition" is not represented in any problems in OG11 or the verbal supplement. There was one OG10 problem that used "concerned at" - but it was in the non-underlined portion, so it wasn't actually testing the usage.

So, I wouldn't call this a "must know" common idiom, but then again, any idiom is fair game and may show up on the test!