Install Titanium Backup and use it to install busybox.
Boot into Recovery. If using clockworkmod, be sure to mount system and data. Using the Android SDK, from the command line, run this stuff:

adb shell
cp /data/data/com.keramidas.TitaniumBackup/files/busybox /system/xbin/busybox
chmod 755 /system/xbin/busybox
./busybox --install -s /system/xbin