Most likely for newer CPU support. But indeed, they have a completely different chipset so after flashing the 1537 bios your laptop WILL be dead...
The 1535 is the original version released back in June ('08). It is based on the Intel 'Santa Rosa' platform- has either the GM965 (integrated graphics) or the PM965 (dedicated ATI graphics). The integrated graphics processor was the Intel GMA X3100. The base processor was the 1.86GHz Intel Pentium Dual-Core T2390 (533MHz FSB, 1mb L2 cache) and the top offering was the 2.5GHz Core 2 Duo T9300 (800MHz FSB, 6mb L2 cache).
It supported RAM up to DDR2-667. The Dell 1397-G wireless card was standard and the 1510-N card was optional. At least here in the States, the Intel 4965AGN wireless card wasn't ever offered (although it is on Inspiron and XPS models).
The 1537 is based on the Intel 'Montevina' platform that was released last July. Dell didn't offer this updated version until just a few months ago (October, if I'm not mistaken).
The chipset for this system is either the Intel GM45 (integrated) or PM45 (dedicated). The integrated graphics processor is the much improved Intel GMA 4500MHD. Dedicated graphics choices remained the same- the ATI Radeon Mobility HD 3450. The 1537 supports DDR2-800 RAM. The Dell 1397-G is still standard and the 1510-N optional, but the Intel 5100AGN wireless card is also optional.
The base processor for the 1537 is the 2.0GHz Intel Pentium Dual-Core T3200 (667MHz FSB, 1mb L2 cache) and options go all the way up to the 2.53GHz Core 2 Duo T9400 (1066MHz FSB, 6mb L2 cache).
The 1537 also gained a combo eSATA/USB port and the standard WXGA (1280x800) display is LED-backlit. The WXGA display on the 1535 was the more common CCFL-backlit type. Facial recognition software also became a new option.