My rt61 device (1814:0301) works fine using rt61.ko in kubuntu feisty. It worked with ndiswrapper 1.38 too but it crashes with ndiswrapper 1.47. My Kernel (2.6.20-16) says: BUG: soft lockup detected on CPU#0! when activating the new kernel.
Hope this information helps to fix this...
edit: I ment when activating the new kernelmodule, not the kernel :)