Hi Rick,
our plans are to add more basic functionalities to the DSP package (improving filter design tools, for example), and then start building other packages that depend on the DSP package for more specialized applications (audio, images, biosignals, etc.). We have just started and can use all the help we can get, so if you want to contribute you will be very welcome!
According to the latest discussion we had in julia-dev, Julia Core/Base will keep having only the very basics, such as filtering, convolution, and FFTs. We'll not have any performance penalties for not being part of Core/Base. We are writing our code in Julia, and most of the Julia Base is written in Julia also. One big exception are FFTs, whose implementation is a wrapper to FFTW, but that is an exception everywhere (even in MATLAB).