I have been having this problem with inability to post to blogger blog, from the date I migrated my blogger account to the blogger beta. I knew that it had something to do with my migration but somehow missed out on looking for a Live Writer upgrade. When I faced this proglem again, only did I remember to look for a Writer update, and I found it. Seems it has been available for sometime. So if you too are having a problem with the blogger beta and Live Writer beta (notice that both are BETA's), donwload the updated Writer version from here: http://windowslivewriter.spaces.live.com/
Happy blogging with Blogger beta and Writer beta.