I have a confession to make, I was wrong. One of my readers made a comment on a blog post asking if the contents of attachments to list items were indexed, and therefore searchable. I said they were not. I was wrong. Had I spent five minutes to test it, I would have known better. After being called out on it by another reader I decided to test it. Imagine my surprise when I found it worked. I had my buddy Shane Young verify my results, and he did. I went to him not because of his rugged good looks, but because he just coauthored a new book on Search (review coming shortly). To sum up, if you attach a document to a list item (a contact, a task, etc) the contents of that file can be found via a search. This is true with both WSS and MOSS. It looks like the filename itself is not indexed, though. You can test this out by searching for "Shazam" on this site. You should get this blog post, a blog comment and an appointment named "Search Test" where I have attached a document with Shazam in it. By contrast, if you search for "b i n g o" (with no spaces) you won't get any hits, even though there's a file with that name attached to the same appointment. Searching for "f o u n d" (again, remove the spaces) will discover that attachment. That demonstrates the attachment is getting indexed, but its filename was not. These tests were all done on WSS, but I verified them on MOSS. With MOSS make sure you run a crawl between when you attach the document and when you search for it.
The one downside I've seen is that the search results link to the list item, not the attachment itself. If the list item has multiple attachments it can be confusing which one is the one containing the phrase you searched for. Right now I don't have a solution to that.
I hope that clears some stuff up. I apologize for any misinformation I may have spread.