Matthew,
Geoff
In your script, you commented "for brevity, left out code that builds csvData..." I'm guessing you're building a string object and then using the "Base64.encode" function to convert that to a file attachemnt object? Will this method work if the source is not a string but rather a file. I have experimented with uploading file objects into a SharePoint document library using VBA and that all works quite well but haven't been able to find similar functionality using JavaScript. Your code above is the closest I've seen to a solution but I don't think it quite gets me there. Are you aware of a similar technique for uploading a file from a local drive to a SharePoint document library? I saw something about using an iFrames object to house/control an Upload.aspx page but I'm not real fond of that method.
Geoff