] Umem cards are battery-backed RAM cards that replace disk
] accesses with non-volatile memory accesses to improve
] response time and maintain reliability in server and
] storage appliances.

This is the special sauce that makes the difference between
a real raid controller and a Linux box. This device is
supported under linux and shows up as a block device. Its
probably really expensive, though. Its just a PCI card with
a bunch of battery-backed dram on it.

Micro Memory's PCI NVRAM Cards