Well, luckily I figured this out on my own. I noticed that the web service method I was having a problem with was returning an object with a collection of LLBLGen data access entities in it. When I took this out of my web service response object ( because I did not need it there ) and rebuilt, I had no problem referencing the web service from BizTalk.
Mental note #1: never pass LLBLGen objects around outside of a business layer.
Mental note #2: never expect BizTalk to consume objects that are questionable in terms of being fully serializable.