My MacBook was stolen: can the thief access my files without my account password?
I'm sorry your MacBook was stolen.  That really sucks.

Sadly, there are numerous ways to read the data ranging from removing the drive or placing the Mac in target mode. Your password only protects things when the mac is booted into your unmodified OS.

To answer your question, Physical access is total access.

It would be trivial for someone to boot using a Linux LiveCD and mount your drive, thus accessing your files.  All they'd have to do to get past your sleep/password would be to hard power down the system.

Additionally, from the link above:


  Mac OS X: Single User Mode
  
  
  
  To boot a Mac into “Single User mode”, simply boot the computer and press Apple + S when blue first
   shows up on the screen. Next, mount the harddrive, and either dump the
   password and crack it with a tool like John the Ripper, or simply
   overwrite the root password:

# /sbin/mount -wu /
# /sbin/SystemStarter
To dump the existing root password:
# nidump passwd
To create a new root password:
# passwd root