You can't edit a blogpost programmatically using office 2007 OM.
I know, its bad.. :( but [I] can't do anything about it. Well..other than letting you know[doing], submitting a bug [done] and requesting a KB [done] .
Bugs are everywhere software, Hardware (and human beings ?) ..even the best of developers do the smallest of mistakes.
Just wanted you to be aware about it so that you don't base your solutions on this assumption.
For the people who are new to it, here is a quick backgrounder (is it a word?):Office 2007 has a blog template, when you load the blog template, you can do everything that you can do form a great blog editor.
Everything here works beautifully.. but there is just one small problem :( AddBlogDocument seems to ignore the postid parameter so "the new document is NOT populated with the contents of the post specified by the value of the PostID parameter" as opposed to what is given in the article.
If you really really need this functionality, I think the only go would be to create a addin and directly communicate with metablog APIs.