imageOpenElectrons, the non-NXT company of Mindsensors, has released a smaller version of their NXShield-M that fits a normal Arduino Duemilanove or Uno.  This one doesn’t have as many servo connections as the NXShield-M (which has an insane number of servo connectors) but has a smaller form factor.

You can attach up to 4 NXT motors and 4 NXT sensors as well as up to 6 RC servos.

You can program it with the normal Arduino programming environment and a library with examples is available from the product page.