Well it seems DeWALT has come out with an adapter that allows you to use the new 20V lithium ion batteries on the old 18V XRP tools.  This is HUGE!  Now you can easily transition into the 20V max line without having to give up your 18V XRP.

Battery Adapter_2-1

The offset design allows it to work most existing 18V XRP tools.

Battery Adapter_3-1

2 20v 2.0ah batteries and a charger (DCA2203C) for $149

Battery Adapter-1

Stand alone for $39 (DCA1820)