You can edit the com.apple.desktop.plist file so your background image will be tiled if it is not already.
Just go into the com.apple.desktop.plist (in your user's Library/Preferences directory) file with pico or your favorite text editor and remove the key for placement as well as the string for it. Take out these lines:
<key>Placement</key>Then save the file, log out and log back in. Your background should now be tiled.
<string>Crop</string>
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011019141650862