Dave Studeman at The Hardball Times gives us an indication why free agent salaries appear to taking off:

Switching gears, let's talk about the current free agent market. In 2004, players who weren't yet eligible for arbitration (rookies and sophomores, essentially) contributed 1,720 Win Shares to their teams. In 2006, that same classification of players contributed 2,669 Win Shares. . . .

At the same time, teams aren't lowering their salary budgets. If anything, they're increasing their salary budgets, thanks to the fine season that just concluded (record-setting attendance) and the cash inflow from Major League Baseball Advanced Media.

See what's happening? Combine more production from inexpensive players with more money to spend, and you have teams with blow-out free agent budgets.