Matt Forte should be able to retain the majority of his PPR value, IMO.
The targets that Marshall will likely be eating into will come from the rest of Chicago's receiving corps.
In 2011 Roy Williams was targeted 63 times (15 games), Johnny Knox 69 (14 games), Dane Sazenbacher 54 (16 games), Earl Bennet 43 times (11 games), and Sam Hurd 15 (12 games)
Roy Williams is no longer on the Bears and Johnny Knox might not play a snap in 2012. Sazenbacher, who did not look like he was going to make the team at one point, will be backing up a healthy Earl Bennet so I don't anticipate their target numbers to be as close this year. Hester's target numbers have been trending down since 2008 and that trend should continue.
All those targets coming off the books should be enough for Brandon Marshall and rookie Alshon Jeffrey.
As far as Michael Bush stealing targets, I don't expect he'll see many. In Oakland Bush shared a backfield with a better pass catcher than him and his receptions were limited because of it. In games that McFadden started last year Michael Bush averaged 1.5 targets per game. Bush is a fine pass catcher but I think it's clear that Forte is the better of the two in that facet of the game.
I know it is a concern that Matt Forte will be on the field less, but he has always done most of his damage between the 20's anyway. In 2011, only 6 of his 52 receptions came in the redzone.
It would be accurate to say that arrival of Bush and Marshall could threaten Forte's ability to score touchdowns, but that's never been where his value has come from. Forte thrives on total yardage and receptions. I don't see why he can't get between 50-60 catches (it would take only 70 to 80 targets to reach that mark) and 250 carries. Forte is explosive enough to turn that into 1500+ total yards and a couple long play TDs which in my mind earn him a spot in the back end of the first round.
Can you tell I just drafted Forte