Our colleagues from Engadget.com have managed to get some info on the XPS M1530 specifications.
According to them the smaller M1530 should feature a 15.4 inch display with CCFL up 1680x1050 and LED up to 1440x900, Core 2 Duo from 1.5GHz to up to 2.6GHz. The chipset should be a Crestline GM/PM 965 with up to 4GB 667MHz DDR2. Unlike Dell's M1730 aka "the beast" which will have a top class 8700M, the smaller brother M1530 will have the Geforce 8400M or 8600M GS option.
This promising machine has HDMI and weighs just 4 pounds or 1.8kg. Of course this is probably the weight without the battery so you should add a bit more.
According to Engadget this new M1530 is planned for November 7th release, but we are talking about Dell so the shiping date might change. You can check out more info here.