So, I had troubles sleeping my Macbook Pro, since I use a Sony VAIO memory card adapter (VGP-MCA20), plugged into the cardbus slot. When I try to sleep my computer, the screen turns black, then the SuperDrive is activated (I can hear the sound), and finally, the password prompt appears, asking for my password to access my session.
I also found this in the System log in Console:
May 10 14:26:12 zvetlana kernel[0]: USB caused wake event (EHCI)
I unplugged then the memory card reader, and then everything seemed to work well. Note that no memory card is inserted in the reader.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060510054904612