there's a nice C++ program and schematic in the Januray 23,2003 issue of EDN -- a PC (parallel port) based instantly programmable analog filter -- uses Maxim DG308a analog switches (you could easily use the analog ADG308's). Author cites some of the shortcomings of digital potentiometers (principally thermal dependency.) Here's the link:
http://www.e-insite.net/ednmag/index.asp?layout=article&articleId=CA269851&pubdate=01/23/2003
http://www.e-insite.net/ednmag/index.asp?layout=article&articleId=CA269851&pubdate=01/23/2003