It depends... did you empty the trash after moving the files there? If not, then the files are safely in the Trash. Open the Trash from the Dock and your files should be there...
If you
did empty the trash after putting the files there, then that gets trickier and there's no guarantee that the files will be recoverable. If they are important files, I would suggest ceasing
all use of the computer immediately, as the longer you use it, the better the chance that the files will not be recoverable (the space they occupied on the hard drive may be overwritten with new files if you use the computer).
In this case, there are several software solutions you can try -- but be warned, recovering an erased file can take hours... the software has to search the entire hard drive for erased files, and that can take a long time.
If your data is
really, really important, I would suggest sending the drive off to a professional data rescue company, like
http://www.datasavers.com. If you'd like to give it a try yourself, you might wanna learn about DataRescue X:
http://versiontracker.com/dyn/moreinfo/macosx/1133
Just keep in mind, the more you use your computer, the less likely you'll be able to recover the files.