In Tiger, if Bluetooth was switched on and your devices were properly paired, you could send files in both directions quickly and easily unless you had disabled it.
In Leopard, this is not the case. File transfers are disabled by default, despite a successful pairing of devices. Worse still, the preference that controls this isn't even in the Bluetooth pane. To re-enable Bluetooth file transfers, visit the Sharing pane in System Preferences, and enable Bluetooth Sharing. Different privileges are available for security purposes.
[robg adds: Although this is fairly obvious, given how different this is from 10.4, I felt it was worth sharing.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071217123516101