Patrick-
IIR filters can easily be implemented using fixed-point arithmetic. The 18-bit multipliers on the Spartan-3 will help quite a bit in that respect. The only real concern with fixed-point in IIR filters (aside from precision) is that they can result in “limit cycles” – noise brought on by the feedback and precision limitations. Fortunately, they aren’t much of a concern for most applications.
Here’s a brief article on that. The Rice Connexions database is really useful for DSP/comm topics.
http://cnx.rice.edu/content/m11928/latest/
The Xilinx mults aren’t designed directly for saturation arithmetic which is often useful in audio applications.