Although the new collective bargaining agreement has not been ratified by the NFLPA just yet (they are in the process of doing so right now), the tentative start dates for training camps around the league have leaked out. Apparently in the new CBA the start dates for training camp are based on the date of a team's first preseason game. 15 days before the first game is when training camp can begin.
The Detroit Lions, which have their first preseason game on Aug. 12, are scheduled to open training camp on Thursday, July 28. Here is a rundown of the schedule for all 32 NFL teams (via Matt Pomeroy):
Obviously nothing is official, but these dates should be announced once the new CBA is officially given the go-ahead by the players.