I would like to echo Walter's informative comments about cuckoo vocalizations. Due to the secretive nature of these birds (and their limited abundance in these parts), I think that many observers, including those with extensive field experience, have problems with these two species. While I think Yellow-billed Cuckoo is well-known for giving its outburst of "kowp" calls (what Walter calls the clatter-call) as well as its mellow, well-spaced "coo" calls, I don't think that many people realize that Black-billed Cuckoo has a second vocalization in addition to its familiar "cu-cu-cu-cu." During a recent outing in Ithaca where both species were seen and heard, I did not hear a single "cu-cu-cu-cu" from the multiple Black-billed Cuckoos detected--they were all giving their clatter call. For a good example of this call (and a comparision with Yellow-billed), I would point people in the direction of Lang Elliott's "Stokes Field Guide to Bird Songs."
Having written all of this, I am curious--Greg and Tom, what vocalization did you hear that you thought sounded like a Yellow-billed?