We're in the same boat.  We've got the projector mounted on the ceiling but can't get the remote/mouse control to function.

Dell sent us a new remote.  Interestingly, it was slightly different from the one we had before.  The new one has an indicator light.  However, we could still not move the mouse using the remote.  We contacted Dell again and they sent us another projector.  However, we can't get this one to work with the remote either.

Before anyone asks - yes, we do know about the slider switch on the remote.

We've tried ps2 cables.  We've tried ps2/usb cables.  We've tried three different PCs.  No luck.  Everything else is working fine - we just can't get the remote to control the mouse.

If anyone has successfully gotten the RS232 connection between the projector and a pc to function - please share some advice or tips.

Thanks.

I also had my remote replaced with a slightly modified one (with an indicator LED). 

This new remote works with the USB interface.  I have the DVI out of my Dell lattitude D600 docking station with the USB connector in one of the USB ports on the docking station and the M1-D connection on the projector (4100).  The mouse functionality works well when it works.