Love the program!
Here's an idea. Once APIs for doing GPGPU calculations on graphics cards get a bit more friendly (especially arbitrary precision - maybe that will be a long time) you should consider doing all the serious number crunching on the GPU. It seems like fractal calculation is just the kind of parallel workload that would do well on a GPU.
Patrick
Yes, with the latest graphics cards rendering fractals using the GPU became quite simple and the results are not bad. However Fraqtive was designed to squeeze the most from the CPU and with some optimization tricks it performs pretty good too :).
Regards,
Michał