Judging from Ken Rosenthal's tweet, it sounds like it's a one-year, major-league deal, which is unfortunate. I assume it won't cost much, but again, if the Pirates non-tender Garrett Jones, who's a superior player at this point, that's a mistake. Of course, there's still the possibility that the Bucs will keep Jones, given that they don't know who their first baseman is going to be, and I'm hopeful that will be what happens.
In that case, signing McLouth isn't a huge mistake, but I'd still probably rather just roll the dice with some NRIs than give McLouth a major-league deal. It's a great move for McLouth, in that he gets to rejoin the one fanbase that will see him for anything other than what he is, which is a defensively-challenged outfielder with severe contact issues. His career is on life support. He hit .190 with the Braves in 2010, then - and this is the really scary part - only batted .234 after being demoted to Class AAA. Then he hit .228 with the Braves in 2011. I'm not saying his batting average is the best indicator of his value, obviously, only pointing out that a guy who provides little defensive value and repeatedly struggles to hit .230 as he heads into his 30s doesn't seem likely to contribute.
Maybe McLouth comes back to Pittsburgh, recovers a little bit of that 2008 magic, and helps out as a bench player. I'll be rooting for him. But it looks at least as likely to me that he joins with Clint Barmes, Rod Barajas and Pedro Alvarez to help the Pirates make a run at the modern record for lowest team batting average. I'm kind of kidding, but kind of not.
Anyway, I'm probably assigning too much significance to this move because it's McLouth, but I wouldn't be a fan of it regardless of who it was.
UPDATE: The Pirates will pay McLouth $1.75 million, which I think is way too much.