If Jackson's issue was his work ethic he is gone for good. The Patriots would be more likely to try and get Doug Gabriel (who also appeared to suffer from that issue), who actually caught a few passes. And, yes, that is sarcasm.
I would take Caldwell back over Jackson in a New York minute. He did drop the pass in Indy, but the week prior he caught the pass against San Diego that allowed the Pats to play Indy and was a reliable receiver during the season. And as Caldwell's visit may be just an emergency roster update as Gaffney (who earned his spot by performing the end of 1006 and 2007) is banged up, it may mean nothing.
As for Jackson, he played in 14 games and has 3 TDs, 13 receptions (average: less than 1 reception per game, somebody else can find who many series he was in as a receiving option for those receptions with his splendid ability to get open). My opinion of him was set when he dropped a pass in stride from Brady his rookie season and seemed to point to everyone but himself explaining the drop. If that was not media nerves and is still his mentality, regardless of his potential, he has no place on the Patriots and hopefully has HD or plasma in his living room to watch the games.