Update via the Globe's Gary Washburn:
Heat coach Erik Spoelstra lists Chris Bosh as a "game-time" decision for tonight's Game 5
No surprises here. I'm certain Bosh will play.
For those looking to stats as a predictor of tonight's outcome, John has done some research.
Since KG's arrival, the Celtcs are 8-0 in Game 5's in a tied series. Only one of those games was on the road: 2010 vs. Cleveland. LeBron churned out 15 points on 3-14 shooting.

Even though Chris Bosh’s return would create more difficulties for KG I don’t think his return would have that much impact. As this article shows this kind of injury takes a while to heal from. It takes much longer to gain full strength than he has allowed himself and having and iffy Bosh back may just cause problems:
http://www.sportsrageous.com/heats-chris-bosh-may-be-back-for-game-5-against-celtics-06-04-2012