So the test was a partial success. After I used the right service address (http://geekswithblogs.net/shartzog/Services/metablogapi.aspx), Ecto loaded my blog right up and pulled all of the posts in. Sweet! However, even though each attempt to post creates an entry (in the future, it turns out)... it immediately reports an error about some type mismatch. The Ecto UI then shows the post as saved / not posted and the post ID is a DateTime value (eg. my test was 20051114194317) instead of the actual post ID.
I suspect that this is an error in the metaweblog API of .Text Version 0.95.2004.102. I doubt this was customized by Jeff, so it was probably a known issue. Hopefully when GWB is pushed to CS 1.1 or CS 2.0 this problem will go away.